According to Google Maps measurements I live just over 700 metres to the west of Lowestoft station, and 125 metres (shielded by intervening houses) and 200 metres clear line of sight (and sound) of the tracks themselves. So with the Norwich and Ipswich services arriving and departing all day long I get to hear plenty of the 755's accelerating out at various rates (especially with working from home!)
The engine note of the "rogue" trains is noticeably louder and does sound akin to a turbofan aircraft engine; and when I first heard it I assumed a new mystery train was about, but no, in to view appeared a 755/3.
Whilst the note is a bit like a turbo fan engine it is certainly not particularly loud, and there are no shrill, or piercing notes. It's certainly not as load as the throaty, window rattling roar of a Class 37 as it left the station. How I miss those!