I too can make a rumbling sound at will since I was a child.

What is responsible for this?

I don't know how I discovered how to do this but it always seemed natural.

Sounds like voluntary opening of the eustachian tubes, also called BTV in freediving. See https://en.m.wikipedia.org/wiki/Ear_clearing

Oh, Thanks. When I was a child I asked all the adults around about this, but they looked at me like I were a lunatic, so eventually I just stopped talking about it.... :) Glad to have an answer now.

I learned to do this as a child because I always had issues with feelings of pressure in my ears. I was so sensitive that riding up or down a hill that was big enough would cause discomfort.

I could imagine a visual form of it being a conditioned reflex. The stapedius reflex is also triggered by vocalization, so clearly some level of top-down modulation is possible.

How do you know?

"Doctor of audiology"?

I am loling, but I think perhaps parent meant to ask "how is it known [... to doctors of audiology]". I.e. how did people determine thay fact. Or maybe he was just being a smartass.

On being a smartass - did you just assume my gender?

</straightface>

Is precisely the person I would hope can explain. Especially given the confidence placed into the negative assertion.