Pretty loud; I have a Naval base nearby and for a sec I thought it was one of their planes overhead. No ballcock in my toilet and it does not stop or let up when I run water, flush the toilet, etc. Plus the toilet is only 3 years old and I had the insides replace about 16 months ago - long story. Looked inside. Not the toilet. Was quiet and fine this morning; ungodly loud tonight. First time and it has not let up.

Are you saying it is a constant vibration that goes on and on and on? What type of toilet do you have that there is no ballcock/fill valve?

Could be the underground plant (heating, cooling etc) for the base.
I heard about a house across the river from a large industrial site, and the rocking chairs started rocking at certain times every day because of it.