I have this strange sound from my car for last 2 months. I installed JBL GTO-3EZ Amplifier + JBL GTO 609C speakers last year and everything was functioning excellent until recently I started hearing this sound trough speakers. The car is Golf 7.
It usually happens once every 2-3 days, few seconds after turning the car ignition off. I even had it 2-3 times while driving. Sometimes it's loud like in the video bellow, but sometimes you can barely hear it. It lasts for ~10sec, and it stopped when I turned on stereo as you can see on video.
I wonder if somehow it could be heat related as we have ~40c weather for last 2 months, as it usually happens after driving in the sun. I think I never heard it at night.