Compressor Noise comming from engine

There is a real faint and quite sound comming from around the throttle body area of the motor. Its sounds like a compressor (ARB or whatever). its really strange. The i took the intake hose off the throttle body and its slightly dirty but nothing that looks like it would make this kind of noise. Anyone know what this might be? How do i trouble shoot this? Its a 99 with a I-6. 62,000miles. Oh also it will make this noise in while the transmission is in park. And its gets louder as you increase RPM. Any info would be great. Thanks

check for crack in exhaust manifold or burn out exhaust gasket.

It's not related to the silencer/venturi, is it? http://go.jeep-xj.info/HowtoSilencer.htm