That sound seems like somewhere in the AMP section a feedback control circuit has gone a little AWOL.
Usually feedback paths are simple resistors that when they go open (as opposed to shorting) the amplifier oscillates just like that noise on the video.
A specialist might be able to pin-point which component(s) has gone faulty, but sometimes not easy to repair depending on how it is soldered to the amp boards.
What I meant by Grounds, was that what the electronics in a car prefers is a common ground, so the lights might all go to a common ground point, and the ECUs (might) go to a common ground point, and the audio systems go to a common ground point. Audio systems without common grounds tend to have noise or instability as small currents move around the grounds points and the amps just amplify the noise.
If you are able to plug in a speaker to the sub amp that is not touching and other metal parts and you still get the noise, that to me indicates the sub amp is the problem.