I had an 4000D with this problem years ago (not audio related). Turned out to be a flakey PAL at location U209 on the A3640 board. A shot of freeze mist directly on that IC would cure it for a few minutes until it got hot again. I seem to remember that PAL was hotter to the touch than it should have been. I couldn't source a new one so I just replaced the board. Might not be your issue but the symptoms were the same.
Jeff
