First, take a rubber mallet to the exhaust system and tap on it. You could have a muffler or catalytic converter that has come apart inside. You can't see the problem, but it will rattle and drive you crazy. As far as the broken motor mount, usually you will get a clunk from that and not a rattle. If you still feel the motor mount could be the issue, do the following:
You will need a helper.
Place the truck on a flat surface and place the parking brake on. Next, open the hood and have a helper start the engine. After that, have the helper first place the vehicle in drive. Making sure the helper holds firmly on the brake, have him give the engine a little throttle. At that time, you watch the engine to see if it is lifting excessively. It should move an inch or so, but if it is more than that, chances are you do have a broken motor mount. NOTE: Do the same thing in reverse to see if the other side of the engine lifts excessively.
Let me know what you find.
Wednesday, August 28th, 2013 AT 5:21 PM