A few years back, while trying to snooze on a Jet2 flight up at cruising altitude, my semi-attention wandered to the droning of the engine. I realised there was a very subtle effect of its intensity rising and falling ever so slightly in a repeating fashion. From memory, I'd say it was doing each rise and fall cycle over around 3-4 secs. It was consistently regular in its pattern. I've never heard that before but it was so subtle it could be it always does it and I've just never noticed. I was intrigued as to what the mechanism might be. Any thoughts? I was in the window seat a row or two in front of the wing at the time if that's relevant to what's audible. Thanks!
That is almost certainly a sound interference pattern due to a slight difference between the spool speeds of the two engines. It’s a much more common problem on reciprocating or turbopropeller powered airplanes, where it can cause considerable discomfort for the passengers, but can also happen aboard turbofan powered aircraft, especially those with high bypass engines.