I had the EXACT same problem about two years ago.
I ended up unplugging the optical input wire to the amp. After that, it never happened again. I suspect it is the head unit. I have heard of several others having their head unit replaced for various reasons.
When I unplugged that input wire, it was glowing red (getting a signal from the head unit). So, one could reason that the head unit was sending an errant signal.
My car was also completely "off". It had been sitting overnight. When I popped the trunk, the LOUD static noise started.
Again, it only happened once....it has never occurred again.
Tim (apparently likes "3" cars)
E90 M3 -- Current ride
2004 GT3 -- Sold
1997 M3/4 -- Sold
1995 M3 -- Sold