Unusual feedback howl from iMac

My iMac (info above) has recently displayed a problem when playing music through iTunes, I do not use any other player. I have placed this question here as I believe there may be a hardware issue.

Irregularly when music is playing and a new track commences there is very loud high pitched noise, sounding very similar to feedback howl. The track may be heard in the background, but the noise is dominant. This happens through the iMac speakers and when using Airport Express through the surround sound system. When it happens the sound will continue into the next track, unless I press pause, then the sound stops but continues if Play is pressed. It will not disappear until a new track is selected. The problem track may be returned to later without any apparent problem existing, which makes me think that there may be a build up of static or a dysfunctional part within the iMac. There does not seem to be any rational reason for this, I have searched various forums but without success.

I play the same playlists via my iphone in the car and on a docking sound system without any disturbance, which is one reason why I suspect the problem may be in the Imac.

Any useful suggestions would be appreciated.

Bit of a soliloquy here.

A few posters on Apple discussion boards have described a similar problem when their iPhones are near their iMacs. Has anybody on here experienced this?