rover 45 ---year 2000 conniseur?

hello
i recently bought this car and am delighted with it.
the only thing that bothers me is when the engine is ticking over there seems to be a very distinct loud humming noise,inside the car.
a friend who knows a bit about cars,says it was the alternater,as the tone of the humming noise changed a little if i activated something.[i.e. the rear window demister]
im still concerned as iv never heard this distinct noise in any other car iv driven
is this common/okay
many thanks