So both a Civic and Integra I've driven makes this noise, although it was a lot louder in the Civic. It usually occurs after driving an hour in the hills, at moderate RPM (3-4K) and light throttle. I would describe the sound as a grumble/rumble/wobbly sound. I don't think it is valves/pistons/knocking/etc because those sounds tend to be somewhat sharp or metallic sounding, and each pulse is evenly spaced out (in time) and have about the same loudness. The sound I am trying to describe sounds kind of "uneven" in terms of loudness per beat. I read over an old usenet post suggesting that it was pre-ignition.
I guess I am being a car hypochondraic because I want to make sure everything is working ok. At least it seems to drive fine and passes smog. It's just that this (and some other engine noises) are somewhat troubling. If I want to take it to a mechanic to have him make sure the engine is working ok, what kind of service should I ask for?
Thanks