Vehicle: 1997 Jeep Grand Cherokee (2WD) <-- :(
Issue: Makes a sort of 'grinding' sound when it's running. It appears to be RPM dependent, and seems to be the worst at low RPMs (idling is the worst). The noise is a sort of grating/grinding. It sounds a little "echoey", so to speak, like something rattling against a hollow chamber. When I start the car or shut it off it is the loudest and scariest sounding, then it dies down a bit. Noise comes from under the car (maybe toward the back? It's hard to tell really over the engine noise).
Ideas: As far as I've researched so far, my best bet is a loose heat shield on the catalytic converter. (I suppose this would explain the rpm dependency if it only makes noise at certain engine vibration frequencies.)
Can anyone confirm this, or offer another explanation? It seems to have worsened as of late. The noise just appeared one day when I started it, although I suppose it's possible it was going on earlier just not loud enough to be noticeable.