I have bought a used Quattro. It seemingly works, but some times it starts to make bad sounds. I tried connecting it with only 50mm dc cables straight to multiple different batteries with no ac connected. Sounds is the same. How ever if it is off for a little it tends to go away again. I made two recordings of the sound: https://www.youtube.com/watch?v=NmyPadE7vqY and https://www.youtube.com/watch?v=CDWX25yAMjA This is not a wiering / battery / ac issue. Any idea what this could be?