Engine sounds from speakers right rear

Some time ago spontaniously the speakers in the right rear door from our A2 Concert audio system started to make a noise; radio on/off no difference. The frequency varies along with the revs. and the electrical systems in operation (cooling fan, power steering, etc.). In the service garage they found out that switching the left and right cables swithed the noise to the left, which proofs that there is nothing wrong with the cables. They also replaced the radio, without result, so ours is OK too. Now they say that it is probably the amplifier under the right front seat that amplifies the signal of the rear speakers. Could be so, does anyone have experience? Before ordering a new one (which I will have to pay, cure or no cure) I thought that there might also be somthing wrong with some of the input cables, e.g. power supply or antenna. Does anyone have a clue here?