a) crappy stereo
b) ground loop causing interference
you didnt tell us what kind of hu, so (a) i cant comment on. As for ground loops, the shop could have been (and most likely was) sloppy with the wires and there is spaghetti back there. When wires cross, especially power and signal wires, you get humming/buzzing/whining.
Does the noise go up when the rpm's go up?