evilbob
Adventurer
[MENTION=97077]iserith[/MENTION] - Hey! I totally used one of your adventure scenarios and wanted to let you know about it! 
For those who haven't seen the link, iserith has a bunch of quick one-shots (and some longer stuff) here.
One of the one shots called Seahorse Run has some randomly inserted discussion here.
I used a different one shot called Quiet Please and thought I'd let you know how it went!
We had 3 PCs at level 3 and I knew they would have an easier time because they all have passive perception scores of 15 or 16 (bard). This allowed them to each search one bookcase per turn without failing, which lowered the difficulty - they completed the entire thing in just 4 rounds. That said, it was still rather difficult!
First, just to make things flow a little more, I had the patron who hired them (a kindly, bumbling wizard) lead them straight to Dewey's private quarters while explaining the situation - all the while getting his voice softer and softer.
Finally he explained they shouldn't make a lot of noise, because there are worse things than poltergeists - which he insulted, and they threw him across the room and into the eastern chamber. This made a huge racket which resulted in a (freebie) "shhh!" and started the fire, which was raging beyond control by the time he reappeared. He then tells them they have about a minute until the fire consumes everything and he'll be waiting at the door wishing them luck - see ya!
I started the poltergeists at their corpses and decided that they wouldn't move much, but if anything was in range they'd swipe it instead of push it - that made it a little easier to figure out where they were. I also decided that a DC 15 religion check (free action) would enable the characters to know they could perform funeral rights at the corpses - another DC 15 religion check that takes one action (and may or may not make noise) - appeasing that poltergeist and instantly "defeating" it. (The players never even tried.) Still, the 3d6 damage from getting tossed around the room plus the 10 damage per smack (done with advantage if they are invisible!) was no joke! One character ended up going down and the others were both hurt; three level 3 characters taking on two CR 2s is well out of their league.
Fortunately, they didn't stick around to fight much. They quickly realized the round table in the middle might have clues in it and went straight for that. They actually used mage hand to open it (ha!) and then nailed the Int check to eliminate half the room - that was probably the biggest roll in the entire encounter, since it saved them so much time. After that, two characters started searching bookcases (hilariously it took two of them 3 turns to find it in six bookcases - they were terrible guessers) while the bard tried to deal with the poltergeists. Faerie fire was used to make one of them visible, which helped a ton since it no longer had advantage on attacks, and then otherwise the bard got beat up a bit trying to keep their attention, and eventually went down. Fortunately they did a lot of the action while getting missed; the "fling them across the room" attack is actually worse than you'd think, since it not only makes noise but also pushes them away from the area they know they need to search! Once they started getting beat up badly, they had already searched almost all the bookcases, and the last round was them getting tossed around as they were scrambling for the door (book in hand).
I didn't keep a strict noise track per your suggestion (I forgot), although if I had they would have gotten 6 marks (3 bard spells and 3 tosses). Instead, each time they got tossed they got a "shhh," which was really more about tension than anything else. 12 marks seems like a decent limit, since if you miss the big skill check it will definitely take you a while, but the whole encounter is still dicey just because of the snowball effect. Basically, the longer it takes the harder it becomes. The poltergeists do pretty heavy damage, and while you actually have a reasonable chance to kill at least one with spells, especially if you have access to faerie fire and magic missile (they're only 22 HP), each spell costs you another tick on the track. I wouldn't even bother making the "failed search" on a bookshelf cost anything, since the two poltergeists are going to be causing lots of noise really quickly anyway. Then again, like everything else in D&D, a larger party is going to have an easier time of it all, so maybe this was only hard because they were a party of 3.
Lastly, I wasn't sure what to reward the party, since you had no suggestions. They didn't defeat any of the creatures, although you could argue they defeated the banshee (within the context of the encounter). At the time I ad-hoc'd 600 XP for the party (just like the Seahorse Run) and the wizard who hired them gave them a choice of one of two level 3 spell scrolls (a fairly appropriate reward according to my understanding of the DMG - and thematic). I figured if they defeated the poltergeists, he would have given them both scrolls (and they would have gotten more XP).
Overall it was pretty fun and definitely nice to have a puzzle that's not just "murder hobo" style. The difficulty seems well-balanced; killing poltergeists can be useful but it's also going to be noisy, and I liked that a few smart skill checks can make things easier. It'd be interesting to see it played with 5 characters who all have +0-4 perception instead of these few sharp-eyed folk. Anyway, thanks for the adventure!

For those who haven't seen the link, iserith has a bunch of quick one-shots (and some longer stuff) here.
One of the one shots called Seahorse Run has some randomly inserted discussion here.
I used a different one shot called Quiet Please and thought I'd let you know how it went!
We had 3 PCs at level 3 and I knew they would have an easier time because they all have passive perception scores of 15 or 16 (bard). This allowed them to each search one bookcase per turn without failing, which lowered the difficulty - they completed the entire thing in just 4 rounds. That said, it was still rather difficult!
First, just to make things flow a little more, I had the patron who hired them (a kindly, bumbling wizard) lead them straight to Dewey's private quarters while explaining the situation - all the while getting his voice softer and softer.

I started the poltergeists at their corpses and decided that they wouldn't move much, but if anything was in range they'd swipe it instead of push it - that made it a little easier to figure out where they were. I also decided that a DC 15 religion check (free action) would enable the characters to know they could perform funeral rights at the corpses - another DC 15 religion check that takes one action (and may or may not make noise) - appeasing that poltergeist and instantly "defeating" it. (The players never even tried.) Still, the 3d6 damage from getting tossed around the room plus the 10 damage per smack (done with advantage if they are invisible!) was no joke! One character ended up going down and the others were both hurt; three level 3 characters taking on two CR 2s is well out of their league.
Fortunately, they didn't stick around to fight much. They quickly realized the round table in the middle might have clues in it and went straight for that. They actually used mage hand to open it (ha!) and then nailed the Int check to eliminate half the room - that was probably the biggest roll in the entire encounter, since it saved them so much time. After that, two characters started searching bookcases (hilariously it took two of them 3 turns to find it in six bookcases - they were terrible guessers) while the bard tried to deal with the poltergeists. Faerie fire was used to make one of them visible, which helped a ton since it no longer had advantage on attacks, and then otherwise the bard got beat up a bit trying to keep their attention, and eventually went down. Fortunately they did a lot of the action while getting missed; the "fling them across the room" attack is actually worse than you'd think, since it not only makes noise but also pushes them away from the area they know they need to search! Once they started getting beat up badly, they had already searched almost all the bookcases, and the last round was them getting tossed around as they were scrambling for the door (book in hand).
I didn't keep a strict noise track per your suggestion (I forgot), although if I had they would have gotten 6 marks (3 bard spells and 3 tosses). Instead, each time they got tossed they got a "shhh," which was really more about tension than anything else. 12 marks seems like a decent limit, since if you miss the big skill check it will definitely take you a while, but the whole encounter is still dicey just because of the snowball effect. Basically, the longer it takes the harder it becomes. The poltergeists do pretty heavy damage, and while you actually have a reasonable chance to kill at least one with spells, especially if you have access to faerie fire and magic missile (they're only 22 HP), each spell costs you another tick on the track. I wouldn't even bother making the "failed search" on a bookshelf cost anything, since the two poltergeists are going to be causing lots of noise really quickly anyway. Then again, like everything else in D&D, a larger party is going to have an easier time of it all, so maybe this was only hard because they were a party of 3.
Lastly, I wasn't sure what to reward the party, since you had no suggestions. They didn't defeat any of the creatures, although you could argue they defeated the banshee (within the context of the encounter). At the time I ad-hoc'd 600 XP for the party (just like the Seahorse Run) and the wizard who hired them gave them a choice of one of two level 3 spell scrolls (a fairly appropriate reward according to my understanding of the DMG - and thematic). I figured if they defeated the poltergeists, he would have given them both scrolls (and they would have gotten more XP).
Overall it was pretty fun and definitely nice to have a puzzle that's not just "murder hobo" style. The difficulty seems well-balanced; killing poltergeists can be useful but it's also going to be noisy, and I liked that a few smart skill checks can make things easier. It'd be interesting to see it played with 5 characters who all have +0-4 perception instead of these few sharp-eyed folk. Anyway, thanks for the adventure!