sounds more involved than I thought. Here's more info:
In the guest bathroom, the same problem occurred 3 years ago and the landlord changed the tank fill valve, he tells me. Since I've lived here, no problem in that bathroom, however, the MASTER bath is where this problem started occurring...I was thinking it could be the same problem, as the toilets are both 35 years old, purchased at the same time. There is NO running of water, etc., so the fill valve doesn't sound like a 'fix'. The noise and vibration is HORRENDOUS...it only stops when I flish the toilet in that bath. If I turn on the shower or faucets, the sound wavers a tin y bit.
Do you still think your original answer is correct?
The outside faucets have not been used for 3 months.
The noise does NOT happen when the toilet is flushed or I use any water in the house...it just starts up in the middle of the night, evening, whenever.
the only time it STOPS is when I flush the toilet in that particular bathroom.