Acura Integra

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?
August 15, 2007.

You could have a bearing on a belt driven part that is going bad or a bad or weak motor mount.

Bruce Hunt
Aug 15, 2007.