There could be some RFI (radio frequency interference) from a low-power transmitter or from a bad LED light, (my TT had this problem) or even from a ground fault in the wiring. If you test the speakers and find no fault there, try turning off lights one at a time, then move on to removing fuses one at a time until you identify the circuit with the fault. Then you should be able to narrow it down to the specific component.
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