Factory Installed Speaker Noise

[DeperLee]

Thanks in advance for any suggestions. My '07 Vibe is equipped with the sun and sound package. The speaker in my rear right door makes an odd, high pitched 'squealing' sound, kinda like you'd expect a tuning fork to make when you hit it, when I go over potholes or bumps in the road. Problem is, it doesn't happen all the time, so when I took it into the dealer to have it looked at, they couldn't repeat the noise even when they drove it over speed bumps. Oddly enough, I can stop the sound by pushing the air recirculation button. Not suprising, if I turn off my stereo, I can also stop the noise. I have an aftermarket receiver installed that allows me to play my ipod through the interface (Kenwood), but the speakers themselves are still factory installed. Thanks again--Jen