So you have an external amp connected to a head unit via RCA interconnects?

You most llely have a "ground loop" - a situation where multiple ground paths exist, with a potential difference (a small voltage difference) betwen the ground locations.

These can be tricky to fix - you could try lifting the ground (shields) from the RCA cable to the amp (at the amp end only) - so that only the centre connection (carrying the audio signal) is connected.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)