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
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