Electrical -- thumping in tail???

[Bobiam]

2001 986.
While stopped in a parking lot ...... parking lights already on........switched on headlights and heard a rapid thumping sound in the tail end. Beats were 2-4 per second and fairly loud...enough to scare the heck out of me. It stopped when I turned the headlights switch off. I then shut the car down and looked out back to see if the spoiler had done something funky. All looked normal and the spoiler operated manually as it should. I restarted, turned on the headlights and all was well again.
The problem has occurred a few more times in a week. Definitely something in the spoiler mechanism.
I have been having a strange problem that is probably related after installing an HID kit 10 mo ago I have my oil and batt idiot lights coming on when headlights are switched on (covered in another post). This appears to be related as it is initiated when the headlights are switched on.
I'm going to undo the HID kit and se if the problems stop, but any advice would be greatly appreciated.
Thanks, Bob