I like "grackle-stuh-ch" (pr like "loch", or for the non-scots out there, like you've just hawked up a loogy) - sounds sufficiently harsh for the duergar.
How, incidentally, does one pronounce "duergar", anyway? I always pronounce it "dwhirr-gar", though I've heard "durgar" often. It could even be "joor-gar".
On a side note, "svirfneblin" always make me think they talk like the Swedish Chef from the Muppets. "Vurdy vurdy vurd".