Just to confirm this theory, I changed the ear pads today, while doing that, removed the white rubber 'cap' type protector on the NC mic and then removed the mic from where it sat. I cleaned out both the rubber part and the removed the small black cotton filter from the mic. Cleaned all and reassembled and they are good as new.

My theory is that the the glue had melted, it looked like syrup when I removed it and this caused the mic to loop into the speaker