Problem with hum on Rear Projection

Hi

Anyone else had this problem?

I have a Toshiba 42PW23P along with a Sony all-in-one DVD and surround sound.

Had a good 18 months of pleasure out of it but recently when watching DVDs there is a hum that changes in volume from bad to v annoying intermittently.

When I turn the TV off (and leave the DVD running) it goes away.

Does anyone have any idea what this could be? It's very frustrating!

Any help very gratefully received!