I've heard from a few sources that one reason modern airliners are quieter is because their engines have a high bypass ratio. The blanket of ambient air that is bypassed shields the exhaust gasses that exit the combustion chamber. Its this blanket of bypassed air that also reduces or blocks the noise, as most of the roar is from the exhaust gases.
The Tornado GR4 RB199 engines have a low bypass ratio therefore sound like variable noise, constant air speed.
Correct
It's where the high speed hot airflow impinges onto the cold slow moving air causing it to shear / rub against each other and creating the noise / crackle, by putting a warm moving layer between them, such as on a high ratio bypass fan engine this reduces the shear effect and hence the noise, on some of the earlier turbojets that has the likes of Stage 3 fitted, they basically added mixers that added more air into the jetpipe to stir it all up a bit and add a layer between the hot and cold.
Chevron exhaust nozzles increases the diameter of the jetpipe to reduce the speed of the high speed jet efflux. You will see those as the serrated edges on the new 787
See
Shelf truncated chevron exhaust nozzle for reduction of exhaust noise and infrared (IR) signature - General Electric Company
Though you will get a lot of high whine noise from the fan on a high ration bypass engine compared to a turbojet.