2001 Jeep Laredo not sure what it is

We have a noise coming from the back some where. Going over a bump it makes a clunk noise that has gotten louder over time. We took it to the local mechanic and he put it up on the lift a couple months ago and could not find anything loose or broken. We kept driving it and now the noise is more often and louder and more of them. I did get it to heppen the other day when I was driving and it made the noise in a long series 15 or 20 of them and slowed down when I slowed down and stopped when I rolled to a stop. It has been doing that more often lately and is really loud because the people down the block were outside and turned to look. I think it is brake related but the mechanic still can not find anything when up on the lift. They think it is more of a body noise like a sensor turning on and off in the back some where. Stumped !