Oh, is that how the brain rumblies thingy works? Huh, I never realized most people couldn't do that.
Yeah, it’s for when your brain thinks there might be a loud noise. I, too, was surprised when I found out that being able to deliberately tense it wasn’t ‘normal’.
"Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear." (Emphasis mine.)
That is: there are 2 methods. The first method only "some...can" and the second method "[all people] can". The second method seems to be exactly what I voluntarily do; unfortunately the second method's description omits mention of voluntariness ("as when" means that yawning is not mandatory) though. Thus, the "all people" method and doing it voluntarily might overlap much of the time.
Aside: I also have a hardly-traumatic "ear popping" (I say it that way because pressure-caused popping is called barotrauma) as a very early precursor to my voluntary rumbling. I wonder how typical that is...
I hope not, because I have been able to control the muscle voluntarily and it is still works after almost four decades.
It doesn't go off by itself all the time though. It's more for sudden loud noise, I don't think it works at all for sustained loud noises.