Engine sounds from speakers right rear

[Maurice]

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?

[Ciaig]

This problem can by a bad ground at the head unit or a bad interconnect cable from the head unit to the under floor power amp or a bad power amp (not a common fault)

The check done at the shop proved only that the power amp right channel output is making noise. A test at the input will rule out the power amp. This interconnect cable is shielded or balanced (twisted pair wires) depending on the audio system installed. You should check the line output at the head unit and the line input on the power amp. I have in some cases needed to replace these lines with shielded audio cable

[Maurice]

Thank you Ciaig for your reply, but isn't it so that it is not proven that the right channel output from the amplifier makes the noise. When they changed right and left they changed the output connectors at the radioside (cable for the right into the left of the radio, and vice versa), and then the noise was directed to the left side.

This does, however not influence your conclusion, that there are two possible problem spots: the cable to the amplifier, or the amplifier itself.

Since I am not a car electronics specialist: how can I test the output signal and the input signal? Can I just make a direct connection (with a cable extension from the speaker cables from the rear left and right) to the output of the radio and to the input of the amp (that is the output of the cable to the amplifier), so I can hear (or not of course) the noise without amplification?

[Ciaig]

OK first disconnect the line in at the power amp and listen for the noise. If noise is heard then check the ground to the power amp. If no noise is heard then check interconnect cable from the radio to the power amp, substitution of the cables is the best way.