Luftwaffe desynchronised aero engines.

[Graham Boak]

This is a matter of adjusting the revolutions of the engines so that the propellors are revolving at the same speed - in synchrony. When propellors rotate at different speeds there will be time when they coincide and times when they don't, producing a beat in the noise, a regular rise and fall. This is generally considered annoying and tiring to the crew, but was thought to confuse sound detectors.