I've had this noise come and go when idling. It sometimes gets louder when the A/C kicks in (with heavier load on the engine). It's harder to hear in a recording from the top of the engine bay, so as you can tell I recorded it from under the car. I'm not sure what is generating the noise. Other than the noise, the car seems to run just fine.

I previously changed the idle pulley (about a year or so before this noise was present). I've read all sorts of threads on TCTs, but am not positive what a bad TCT makes the engine sound like.

I had my local shop take a look at it, but the noise isn't always present and as luck would have it, it wasn't present twice when having them take a look at it. I finally got a decent recording of the noise tonight.

Any ideas / clues would be appreciated.