It was just occasional at first, now it happens every time a toilet is flushed or water is used. At our main water hook up to the house a loud vibrating noise occurs. Wht can this be?

IT'It is called Water Hammer. You can stop it by installing a pressure reducing valve in the supply line. I found this site that is informative as well.

Howstuffworks "How To Stop Water Hammer"

Loose washer in the main shut off valve, a defective pressure regulator or trash in the pressure regulator valve if you have one.

I frequently agree with Amricca but I think a loud single noise is a water hammer, this does sound like a loose washer, likely in the incoming main if I had to pick a place to start.