My wild guess is that you are getting a sympathethic harmonic resonance at certain times/conditins as you are driving the car.
I had a rattle at certain speeds in my RA23, could never find it when it was parked/static.
Jacked it up solidly on all four wheels at one end of a large empty car park, had a volunteer sitting in the car ready to kill it if things went wrong..... and gentley changed gears and played with the throttle until the rattle appearred.....and sure enough it was the top half of a Boge shock on the rear.....ratttling against the lower section. I don't recommend testing this way, but I found my problem when thousands could not.