Ihave the same issue on my 53 X5, that the dealer never managed to reproduce, even though I get it loads. Apparently I have been told it might be a faulty amplifier (a hairline crack on the mainboard) and it needs replacing. I find this out now that the car is not in warranty and the dealer doesnt want to know. I get the popping noise everytime its a warm day.....

I still have the issue...

Andrew