In my time at the horsey railroad I was involved in various union pushed efforts to investigate/reduce the noise on the AC44C6M units.
The AC44C6M are signficantly louder in the cab than the Dash 9 units they were made from. Additionally AC44CWM rebuilds like the CP 8000 units are bad as well. Much louder than the AC44s they were rebuilt from.
The answer I was able to eventually figure out was that the orignal Dash 9/AC44 had a signficant amount of insulation in the cab walls and floor to deaden the sound from the prime mover, which on FDL equipped units is mounted directly to the frame.
When the Evolution series units came out, GE isolated the prime mover/alternator from the locomotive frame. This move cut down on cab noise signficantly.
At first the cab design was left alone, these early ES44 units are exceptionally quiet. However when the cab design was changed in the late 2000s GE changed the insulation design of the cab. Removing much of the underfloor insulation.(when the nose door switched sides) I'm asuming that GE deemed it unnessicary because the prime mover isolation did a fairly good job of reducing noise levels.
Fast forward to roughly 2015-2016 GE starts doing the C6M/CWM rebuilds. In this program the units get new cabs and electrical cabinets of the latest ES44 design. These cabs do not have the insulation and sound deadening of the cabs that they replaced, and as such are MUCH louder.
The early units are the worst. NS 4000-4001 are exceptionally bad. As the problem was recognized there were some changes made to quiet them down but they are still louder than what they were as Dash 9s and AC44s.
As I have been removed from the frieght side of things for a bit I don't remeber the actual numbers for just how loud the rebuilds were, but it was slightly worse than the non-isolated cab EMD SD70M-2/ACe