PROBLEM IDENTIFIED / SOLVED - KRK Rokit G5 (and likely earlier models)
I had the same buzzing issue when playing low/middle D notes on instruments like Rhodes piano and synth or natural bass. The cause was a loose screw on the back of the speaker panel.
By pressing on different screws, I identified that the CENTER SCREW was responsible for the rattle at higher volumes, particularly around the D note's resonant frequency.
Tightening or loosening the screw didn’t help, but removing the center screw completely fixed the problem.
Give it a try if you're experiencing the same issue!
Hopefully this helps someone.
