Love to know if anybody has experienced this or has suggestions on the resolution.

This is my wife's car and here are her notes:
Began in April.
Periodic episodes
First was loudest longest
No predictable pattern
Seems to be electrical sounding coming from inside convertible top
Description:
Electrified Pop Rocks x 10 loudly bouncing and popping inside and across roof
Described by one witness as "sounds like firecrackers inside roof"
Sound system periodically stops working, then in a day or two returns
Can last for a few minutes to 20
Loudest staticky noise I ever hear

Certainly seems to be electrical. I have not heard this personally but others have. My mechanic looked at it and did not find anything. He checked the Mini service bulletins but found noting like this problem.

Any suggestions?