I just received a pair of HD 560S, but I’ve had an HD 599 and PC37X for years with similar construction, and I’m having some trouble imagining the “quiet, dull sound” which you’re describing as your bigger problem.
If you flex the plastic, it may creak. I can ask if some sort of lubricant might help reduce rubbing and friction sounds, but you don’t want to experiment with potentially shorting out any electronics.
Normally, the plastics will polish smooth over time and use, and the snug fit should prevent the headphone from flexing and moving around while you’re wearing it. You might want to check that what you’re hearing isn’t microphonics from the cable rubbing against your shirt; not usually a problem with smooth rubber cables like the stock one, but cloth wrapped cords (especially if it’s coarse cloth) greatly amplified this.