Regciv

I'm in a four-door '09 bone stock base model Civic. I've put just shy of 40k on it so far, but now it's being weird. It's developed an intermittent sort of rattle (nearest approximation of the sound, it doesn't sound like any rattle I can recall hearing, but I can't think of anything it sounds MORE like).

It began nearly a week ago, and it was quiet enough that unless the A/C was on low and the radio was off, you wouldn't hear it. Now you can hear it clearly with the A/C on high and the radio on medium volume. Its pitch and volume aren't related to engine RPMs, vehicle speed (or whether it's moving or not), fan speed, or the roughness/smoothness of the road. Its volume has steadily increased ever since I first noticed it, though its pitch seems to be the same.

It seems to be coming from in front of the firewall, and it seems to be louder on the driver's side. It begins suddenly, and ends instantly, without any fading out or readily apparent spinning down...so I'm guessing it's probably related to something that isn't always running, like the engine fan. Except that in the Texas summer (it's been over 100 degrees every day for over a month now) the engine fan's running much more often than I hear this noise.

Does anybody have any idea what might cause this?