Took a friend for a drive with me and he came to the same conclusion that I had.
This odd sound is similar to an audio and power cable being run together.
I used to come across this years ago as a kid from installing our own car stereos.
Back OT however, here's a picture showing what
onfireX5 first picture would look like laying down below the amp looking forward.
This is the outside edge of the car on the left, showing the ASD cable unplugged and the LHS heatsink - to the right of that is the amp.
