Interference noise front left speaker

Has anyone experienced this? I've just come back from a trip to the French Alps, the car was flawless apart from one niggling problem. Whenever I breach the 75 - 85mph (approximately) speed, the front left speaker either rattles, or more likely, emits a kind of interference noise, almost like an FM radio that is not tuned to a station, or it's picking up electronic interference from somewhere (I did have a full boot, including laptop (powered down)). I also asked passengers to turn off their phones as a test - no change.

This wasn't ideal on a 900 mile journey, but when the radio/media was on at moderate levels, it was tolerable.

If I'm the only one with this problem, I imagine the trouble shooting is going to be pretty much impossible!

This is on the car according to my signature:

2013 vRS TDi DSG combi with Canton.

Thanks.