Originally Posted by GMB14U2NV

I would check the front end for missing plastic fasteners and or bolts. In particular the 3 center plate/guard fasteners. If the 3 center points are not present - the cover will bounce against the under engine frame and resonate to the mount points sounding like middle of vehicle.

If it's aftermarket under shield - get some heavy duty fasteners that hold the center - 3 of 'em. (AutoZone Medic Isle - package of 4 is what I saw)

If you only have the outer edges of the plastic under shield or aluminum shield fastened and not the center - my friend - it's gonna bounce against the sub frame. I learned this after doing my first oil change and thinking I was slick by leaving the 3 fasteners out for "next time...."

Got to 80mph on a toll road here and thump thump thump thump..... then when I slowed down thinking WTF???? it stopped. After the second day of the same sound, I put the center 3 back in and have never heard the thumping since....