bosch classix tumble drier buzzing sound?

It is more than likely that the capacitor has ceased working. It is fixed to the side of the motor and stores energy to smoothly start the drum at the start of each rotation.
Its usually fixed with one nut and has a couple of wires going to the motor. If it looks swollen in any way-replace it, else get someone to test it. Mine cost a tenner and saved me throwing away a perfectly good t/dryer.
Have a look at this place for spares: http://www.spares4appliances.co.uk/home-laundry-spare-parts/

Geoff in Wales
February 2012