A37575, and I. lived under many a flight path of German Bombers of all kinds in 1940 and 41. I can, as, he does, recall the unsynchronised beat. It would look like a 50 cycle sine wave on an osicillorscope I expect. Anyway, I am not in a position to offer a positive view that it was deliberate. The Jumo engines on the He111 was always defineable.
However, I am not sure I buy a deliberate attempt to unnerve us on the ground. Just HOW would large formations as in August, September and October - and beyond - of all types all de-synchronise to make it worthwhile. It didn't bother me that's for sure.