I have a stock 07 Focus Se 4dr. Bought in may from Carmax with 19000 miles. In July it started making a kind fo bumping noise. Thought it was the spare siince it wasn'r secure. I was layed up for awhile and couldn't look at it. it started again more often around 60MPH. Took it to Ford and they couldn't dupilcate. # weeks pass and it gte worse. Take it in today, mechanic rides with and here's it they put a different mech on it and he notices the rear bumper covering has been replaced, (Carmax didn't tell me this) and it was loose and MUST be the problem. I bring it home tighten everything add sound deadening material every wher back there. drive it and no change. Ford says bring it back, they charged the last time because the replacement bumper isn't under warranty. The noise forst sound like a backfire to me, but it now sounds more of a banging or popping noise. Have to get to 60 or above for it to do it. Still sounds like a popping or something hitting metal. Any guesses?