Certainly could be partially that but as Bob C posted...individual a/c used to do glide approaches and also changed throttle settings to evade/confuse sound detection.
Peter Stahl refers to these procedures in 'The Diving Eagle'

Was not the continued de-synchronising of engines to confound acoustic detection a bit of spin to obscure the discussion of radar detection?

The synchronising of an aircraft's propeller speed is practical and desirable: comfort, vibration, human and equipment fatigue etc but synchronising the props of one 'plane with those of another is not possible. Thus a group of aircraft within the range of a detector would sound the same to that detector whether or not each aircraft had synchronised engines.


Some interesting pictures and some notes about acoustic aircraft detection here
Acoustic Radar.

Horsefern
What a coincidence! I too lived in New Malden, during the Blitz and recall quite clearly poking my head out of the Anderson shelter to listen to the bombers...that is until my mum yanked me back in side and gave me a clip round the ear!
It always mystified me why the sound was different, hence me starting this thread. Many thanks to all those who replied with interesting answers

I wondered if the out of sync noise was intended to terrorise the civilian population? I know it did.

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.

The Luftwaffe aircrew had little idea of the capabilities of our 'Radar' system in the early days of the war and so certainly some solo bombers did alter throttle settings in an attempt to confuse detectors...

From Peter Stahl 'The diving Eagle'...
Target Liverpool march 1941 ...Heavily engaged by searchlights...
''I try every trick I know to mislead them,but in vain,every time i change direction they follow me.Even the usually effective method of deceiving the defenses,by changing the revs of the engines to alter their sound,fails this time''.

Who knows...inland it may have been effective because I doubt there was much inland radar coverage early in the war.

Actually - looking at the date of the quote from Stahl's book - possibly it could have been an early encounter with the new (inc inland) GCI system which was coming into service in the spring of 1941 !