Porsche Cayenne - Intermittent Audio Weirdness

Some friends have an '09 Cayenne (diesel). An electrical gremlin is periodically causing two tones to play in the speakers: one is a constant drone, which sounds remarkably like the drone of bagpipes. The tone doesn't change with rpm or road speed. The other is a high-frequency tone that changes with rpm; it's quieter and sounds like a typical suppression issue.

Turning the stereo off doesn't stop the tones, nor does changing the volume affect them. 

The tones come and go randomly. The drone tone is loud enough to be annoying. 

My friend said he wasn't able to find anything online about this. 

Before he takes it to a shop, does any of this sound familiar (no pun intended)?

Sounds like a sound symposer issue.  Not sure if that vintage Cayenne came with noise cancelling/symposer.

Does he have a bluetooth adapter of any sorts plugged into the car?  

It looks like the '09 had a sound symposer available but I wasn't able to find if the diesels had them. Looks like it only works in Sport mode - I'll ask if they leave it in that mode. I didn't find fuse info. Also, who knows if it matters, but it's a German-market car. 

I'll ask about Bluetooth.