I've been on a similar quest to quieten my UM2E:
Swapping all the fans for Sunon Maglevs helped my ears stop ringing.
Padded feed helped a bit, but I suspect print quality is suffering as a result.
Putting a door on the front and a lid on the top was a complete waste of time.
Enclosing the whole thing in a box made of 16mm 5 layer polycarbonate sheeting made a massive difference - it's used for greenhouses and blocking road noise. I can sit next to it while printing and hold a phone conversation, but I can still feel that stepper hum in my bones.
The polycarbonate case does an awesome job of trapping heat for ABS printing - perhaps too good. I have a pair of 60mm fans, a thermistor, a handful of LEDs, an Arduino and a half baked plan to create some sort of self-regulating cooling system + pimp lights for the enclosure.
I've also ordered a set of stepper motor dampers which I hope will fit and finish the job: https://www.matterhackers.com/store/printer-accessories/3d-printer-motor-vibration-dampers-4-pack
If that doesn't do the trick I'm at a crossroads - do I replace the PCB and stepper drivers or do I look into yet more sound isolation? If someone has some experience with this I'd really appreciate the advice. I've been going through the forum looking for info, but I don't know much about the subject to begin with so going is slow.