I have a 98 Civic LX that I picked up used with about 50k on it. Since picking it up, it has been having a strange noise at about 3k on up and in any gear.

It has been stumping me on where it is coming from because I cannot duplicate it while not moving. But the problem is definately not driveline related.

The noise sounds something like an air pump. It's sort of deep sounding and increases with engine rpm. The sound is not there when I first start the car, but as it warms up it can definately be heard.

What I am thinking is that it is the catalytic converter or something in the exhaust.

Any ideas or recommendations for me to check?