Need a hand with wife's trucking, thumping noise.

New member here, could use a hand. The wife loves her truck, havent had any problems out of it for 60K until now. When she cranks the truck to start it, you can hear a thumping noise that sounds like its coming from the passenger side or the trans tunnel. It also makes the same noise below say, 1300 rpm or so while maintain the speed on flat level ground (light throttle). As soon apply more pedal and it down shifts it goes away. Almost sounds like someone is thumping on something with a rubber mallet. Have there been any split engine/trans mount problems or anything?

Im gonna get under the truck today after I take the log splitter back over to my dads and get some room in the garage and fire up the heater, its cold out today. Was just hoping for a place to start, thanks guys.