Very thorough response. Sadly I think I've already done most everything I can. I already use third party fans and run them at a low rpm. I have a dedicated ductless AC just for my gaming space which is itself whisper quiet. If anything my experience with closed back has helped me appreciate just how not bad a time I was having with the open backs I was already using. It might might be because I'm autistic, or because I have OCD, but the groaning and creaking of my left jaw muscle is actually so bad in my left ear it starts to cause mild pain. I don't think I could ever get used to it. Makes me appreciate the soft, consistent hum of the PC by contrast.

The fact the problem is only in one ear makes me think you might be onto something when it comes to TMJ. The problem with the Mezes is you can't really change the clamping force at all. They're elastic and I'm not sure I want to risk damaging them by trying to stretch them out. I'd consider looking for different pads for them, but anything less isolating would kind of defeat the point.

The one silver lining is they seem great for listening to music, since that more consistently drowns out the jaw noise.

Thanks for your help. If you think of anything else, please let me know.

Edit: I should clarify I don't think I'm hearing my heartbeat. I'm hearing rapid pulsing whenever I move my jaw, on my left side. This does not seem like vascular noise.