I have searched the forums but am not coming up with anything that sounds similar.

It happens with engine running, in any gear, and is not dependent on the car moving. I have noticed that the pitch lowers just slightly when gas is applied. I notice it the most with the radio off, while in slow and go traffic: coasting along it is just audible, when I apply the to stay with traffic the volume stays the same but pitch decreases.

Oddly I have also noted that after I park and shut the engine off it continues for a minute, decreasing.

My two uninformed theory's are: 1. any of the vacume actuated ventilation devices could be leaking and causing this? 2. something in the exhaust?

I should be able to ignore this, but its bugging me, like a faucet drip in another room or something.

so thanks for any ideas.
n