Engine/Speaker Interferance

[sd1800]

I have just noticed a issue with the front left speaker. It will happen regardless of whether the radio is on or off. It basically whistles or makes a hi-pitch whine noise at idle. It gets higher pitched if I rev from idle up to about 2000rpm after which point it goes away but will then return when the engine returns to idle.

It will not to do all the time but is rather a intermittant problem.

Completely stock & untouched AU1 premium sound (with the stacker in the boot, not in-dash).

Any help would be much appreciated.