Whine noise & temperature fluctuations in power shower?!?

The Armitage Shanks mixer unit on my power shower has developed an annoying whining noise together with fluctuations to water temperature. The noise can sometimes be stopped for a few seconds if the adjustment lever is turned towards cold- but soon returns. Then, sometimes, it doesnt make any noise!!
It is a traditional, barrel shaped chrome mixer unit, about 9 years old. Any thoughts??