What is causing continuous bumping sound coming from underneath

Have someone who knows what the hell that they are doing look at it. If it's the steering you and everyone else is in grave danger

In my opinion, I would say that you should check the Muffler system from the engine all the way back. You can do a good deal of that looking by yourself, starting with the tailpipe. Try to move it, when cooled off to touch, by 1st pushing on it and then by swinging it side to side and up and down.