I've often as a passenger noticed the difference of the sound coming from different airliners. I believe the two manufacturers I have compared are Rolls-Royce and GE, where the latter's engines sound more "electric" (funny enough) and the former's more distorted and noisy. Do these observations make sense? And what could possibly create such a difference i sound? I was in the belief that engine design of turbofan engines is somewhat established and detailed.
The majority of engine noise, especially the "ripping, howling, typical jet noise" comes from the engine core.
The higher the bypass ratio, the less air passing through the core for any given thrust and the more cold air meets the core exhaust at the back reducing the shear
Jet engines with no bypass ratio and deep intakes produce a wonderful sound
Intake resonance - particularly evident on the Avro Vulcan. It is a sound that speakers can do no justice to.