I'm not sure if I'm just now noticing this issue or not, because I'm finally starting to drive my car more often. I had the car for 4 years and only put on about 4k miles so far.
Anyway, I've been noticing that during idle there is a tap or rattle sound. If the revs go above 2k, the sound goes away. Not just because the exhaust is loud enough to cover it up, but it goes away. I'm not sure if it's coming from the engine bay or from the exhaust it is hard to pin point. Almost every time I press in the clutch the sound comes back momentarily when the rpms drop if I shift slow enough, so it's not a warm-up issue either.

 

Probably a heat shield. But you might also check the metal pans under the transmission and right rear suspension. I've had both make noise at certain resonances.

 

Got cruise control? If it's not a heat shield, it could be to blame. On my 94 base, the cruise control servo assembly (near the brake master cylinder) has a buzzy rattle at idle. In the car it sounds like it is coming from the dash gauges or firewall.

With the engine off, open the hood and tap on the servo bracket to see if you can duplicate the noise.