Dolby ProLogic surround sound

[CharlieAlpha66]

My C70 cabriolet's Dolby ProLogic surround sound system is outputting a high pitched tone when in Pro Logic mode (noticably not when in 3CH or the other of the three modes).

This happens whether listening to a CD or the radio. I am guessing it is some kind of feedback/interference with the central loudspeaker.

Has anyone else had this problem? Is it a known 'feature' of these systems? If so, what can be done about it and is it a dealer solution? (the car is still under manufacturer's warranty)