Oh wow, my SO has had sleep issues over a long period of time, where she claims there's a low rumbling noise, like a truck idling far away, preventing her from falling asleep. She's talked with her doctor about it but so far nothing. However she is being treated for having a clenched teeth while sleeping...

Just the other day, she noticed that tilting her head in a particular way made the rumbling go away. To us both this indicated that the rumbling must be coming from within, and not an external thing.

Could it all be her jaw?!?