When I pair my iphone via bluetooth there is a hiss / hum in the background. (start up in cold weather)
This noise is present only for a few minutes and then goes away.
It's a continual hiss for a few minutes, it becomes intermittent for a few seconds and then it goes away.
Once the hum is gone if I stop the car and start it again the hum doesn't return
During the first minutes, I've tried everything to find the source without any success.
I turned off the fan, turned off the heating, restarted my phone, etc.

Any idea what it could be?

Thanks guys