I have a fog horn noise that emenates from the inner pipes well after the toilet flushes; sometimes long and sometimes short, but doesn't come from the toilet because it happens after any of three toilets in the house flush.

A long time ago I had a call with symptoms like yours, Only this one kicked in at 3 in the AM and scared hell out of the customers. They thought their house was haunted. It was hard the track down because the water picked up the vibration and carried it throughout the house. We call that sympathetic viberation. Take your hand and run it around the flat part of the flapper. Are the edges a little wavey? If so you have found the culprit. What's happening is the water seeping past the flapper lowers the water level in the tank and the ballcock attempts to compensate. In time the water seeping out and the water coming in will balance out. The ballcock can't shut off and the sound you hear is the washer vibrating against its seat. The repair is simple. Replace the flapper. A few installation tips. The linkage between the flapper and the flush lever should have 1/4" of play when the flapper's seated. The water level should be 1/2" below the top of the white overflow tube when the tank is filled. Hope this helps, Tom