Whining noise coming from back on my 316i e46 bmw

Hi, there, Check your break pads. Could be from there.
cheers

If its a rear wheel drive car it could be your rear differential out of fluid or depending on miles it could be the rear differential gears are about to go. Usually when you hear a whining it will speed up and slow down with your speed. I would check your rear fluid first though. You can also pull the rear diff cover and look at the wear of the gears. See if there is a bunch of metal in the housing. Might need a new set of gears.