Hissing, eh? Not sure. My first guess would be a leak in those tubes in the ears?

Definitely go see an ENT guy for this one.

Thanks for taking the time to reply.

I had all the top teeth crowned and I wonder if there is a bit of tension across the teeth that is relieved when I press the rear of the top teeth.

I'm going to try to get the original dentist to separate the top teeth just a tad to see if it has the same effect as pressing them from behind.

I had thought about a small hole in the ear drum and will pursue that more fully now that you've brought it to the fore again.

Thank you.