Hissing & high pitched whistling noise

Evening Gents,

over the last couple of weeks when driving I have become aware of initially a high piched whistling sound which has now been joind by a hissing noise. Both noises are dependant on engine revs. For instance they increase in frequency as I accelarate or slow down.

Both noises are not noticible when the engine is ticking over.

Anyone got any bright ideas other than switching the radio on?

Car is a 2001 1.8 Zetec petrol by the way.

Many thanks.