[elliotm]

Got these back from being serviced under warranty.  Just noticed after someone in-game commenting that my mic had some low hum line noise on it.
Wanting to circumvent waiting for through a 3 month turnaround for servicing them, again, I popped open the ear with microphone mounted and took a shot of the wiring.  The MMX300 is pretty much a DT770 with a built in boom mic.
 
Upon turning on mic monitoring it's sounds like a kind of hum you'd get from a grounding problem.  I haven't "seen" it before, but I've heard what it can sound like.  Visibly, I'm not sure what to look for, though I find those long lengths of bare wire suspect.  Should I try covering them in electric tape?
 
Tested with another headset (Senn PC350) and no issue.  It's not the sound card, either (Asus XonarD2X).
 
Just looking for guidance before I act, or otherwise start a new support ticket with Beyerdynamic.

 

[Schnens]

I found the solution for my problem with the low hum noise. After literally years of searching for a solution, opening my headset, isolating wires, testing different soundcards... I got the idea from a reddit topic (https://www.reddit.com/r/audio/comments/5s9okb/microphone_emi_interference_beyerdynamic_mmx_300/):

"It buzzes much less if I touch my PC case (metal)" -> this also made it much better for me, so I did the following:

Change one lacquered (painted) screw holding my PSU (power supply) on the case with a bare metal screw. This fixed my electric grounding (electric mass) issue and I never got the humming again. Hope this helps someone who may be as desperate as I was. I will leave some more search terms for the search engines:

Static noise, hum, humming, buzz, buzzing, Beyerdynamic MMX 300, microphone, mic, headset