I have a 1999 Acura Integra GS-R with 1.8 liter engine. It has about 57,000 miles on it. Over the past 4000 miles I have noticed a pronounced vibration, kind of like a distant helicopter, whenever the engine is between 2300 and 2500 RPMs. The car has a manual transmission, but the problem seems to be coming from the engine since it occurs when it is revved in neutral. I have tried to listen for the culprit but I can't pinpoint it except to say that it seems to be coming from somewhere in the engine block. Any idea what might be causing an RPM specific vibration?