Beeping noise with ANC on? Likely the mesh came loose.
Glue they used in factory didn't last the test of time, and proceeds to come off, leading the ANC software to freak out, and beep.
Unfortunately Sennheiser didn't own up, and do a recall. They did address all the issues and sensibly made small tweaks to MTW4s to make a it more refined product.
Comment below if this worked for you, and if someone from Sennheiser sees this, please do something about it officially.
