I can do the rumbling too. And a clicking too. I can even make someone else hear my ear clicking by having them press their ear to mine. I wonder if that's causes by the same thing.

Same here, I hear a rumbling. I do occasionally use this to block or lower very loud (potentially hearing-damaging) sounds if there are no other means available.

According to the Wikipedia article it may be involuntary.

Same. Wonder how common this is.