PMN – The 757 sound is a result of the RB211-535E4 motors they use. They are a 3 spool engine whereas most turbofans these days are 2 spoolers. The 744 use the same units but the sound is somewhat different as there are 4 of them but you can still pick out that underlying tone.

I have heard a similar sound on the 744, but like you say, it isn’t quite the same.

Thanks for the explanation!

Paul