I recently bought the Massdrop x AKG K7XX Red Edition for $200. They're an amazing sounding pair of open back headphones and are frequently heralded for having good sound stage. These are replacing a four year old pair of Sennheiser HD 25 II's which still sound great but can lead to ear fatigue.
The only problem with them is that they squeak like crazy. There are hundreds of online reviews where people complain about this problem. I'm able to reproduce it by merely turning my head or clenching my jaw. The sound is loud enough that other people in the same room notice it.
Luckily the fix is pretty easy. Just grab a sheet of black paper from your local art supply. Cut the paper into thin strips about 1cm wide and 3 or 4 inches long.
Once that's done insert the paper into the gap where they rotate. For my headphones I had to insert them on the back side. With your particular pair you may need to try a few different locations until you find the sweet spot.
Originally I used a sticky note but the manilla paper was too gaudy. It's bad enough that a $200 pair of headphones require such a silly hack. Black construction paper does the trick and is nearly invisible.
The sticky note I tried was very thin and worked fine. The construction paper I'm now using is fairly heavy, being rated at 92lbs. I would avoid card-stock but anything else should work.