It sounds like the engine is falling out. I have looked to see if I could find anything broken but no luck. If I could locate the cause I could fix it. I had a similar sound once from an old car that had a broken universal. I ignored it until it let loose and if I went over 10 miles an hour it would slap the floor. Checked all the universals but they all look in tack. The problem occurs during acceleration and goes away if yuo coast. It is amazing that we had one of the first explorers that ran like a top until we traded it in after 165000 for this one. I wish I had the old one back. Ford is the only car company who's car get worse with age because they find out what works and then get rid of it for something cheaper. And they wonder why we are in trouble. It is also amazing how much corrosion the car has for only 7 years. Thats another reason why american car makers are loosing customers.

I had a similar problem with our 2001 Explorer. Turned out the 4x4 was engaged. The electronic module on our Explorer blew back in January. I was told by our dealership that the module was not necessary if I did not plan to use the 4x4. In August, the thumping began. The dealership told us that even with the electronic module shot, it will still monitor and/or send signals. And sometimes it will kick in your 4x4 High. The thumping is your drivetrain and differentials attempting to gain traction and when it is on a hard road surface there is no give. We had to replace the module, problem solved? Not really, we were told that there may be damage to our transfer case or differentials because of this electronic glitch. Well I can report that our front differential is making all sorts of noises now. Thanks FORD - Found On Road Dead!