the players would tap walls looking for secrets and the DM would tell them when they might have found a secret door without regard to die rolls.
I first learned about the concept of finding secret doors and pit covers by tapping with a pole, (or something), here on ENWorld, recently. During my BD&D and AD&D gaming years, I thought the way to find a secret door was the DM rolling a 1 on 1d6 after the Player said he was searching. That's what the rules described and said; I never saw anything that explained rapping with a pole to find secret doors.
I had always pictured/imagined secret doors and pit covers to be constructed of the same material as the rest of the dungeon -- usually stone, (like this Tomb). And I never thought that secret doors might be made of different stuff, or might be different thickness, or whatever, than the wall it was mounted in.
It never occurred to me, and no one else ever told me, that tapping a secret door/pit cover might make a different sound than normal stone walls. I thought a 10' pole was used in hopes of "tricking" a pit trap to open before anyone stepped on it, not to discover sound differences.
I knew/could imagine many uses for a 10' pole, but direct secret door detection was never one of them. <shrug> Maybe my imagination was weak. Maybe my knowledge of engineering was lacking. Surely I'm not the only person to not know of these concepts.
So, if I had run this adventure back in the day, the Players would have had to get a 1 on 1d6 to find a secret door. If they had gone around tapping the walls, I would never have mentioned that they hear a difference while tapping on a secret door. Not because I would have been hardass or anything, but just because I had no concept of "secret doors sound different."
Bullgrit