Over the last couple of days I've noticed a weird noise from my VX. It is kind of a random ticking or chattering sound, kind of like what a small fan makes when it's lost a ball-bearing - or the sound a hard disk makes when it is doing a lot of seeking.

Some characteristics:

1) Only seems to occur after the engine has warmed up.
2) Does not happen when VX is at a stop - I even put it in neutral and revved it with no result.
3) Not really affected by acceleration or coasting.
4) Can't hear it if the stereo is on medium-loud.
5) Seems to chatter consistently as long as engine is warm and vehicle is in motion.
6) Not the heater fan (I've turned fan off and played with just about all of the temp/ac controls without making a difference).

Any ideas as to what this might be? I don't have my S/C installed yet, so it isn't that infamous ticking sound either...