TJ groans when he lies down–It's not his pancreas, liver or anything I can see.

So for the past 2 months, when TJ lies down (he’s 3 by the way) he groans. It sounds like he may be in pain. He doesn’t do it all of the time, so I was just observing him. I took him to the Vet to get blood work done and everything checked out OK. She thought it was the pancreas, but it’s not, thankfully. In 2 days the Vet wants to do Xrays. I am not to let TJ go up or down any steps, not even 1. TJ is so full of life and energy. Yesterday he leaped over a street barrier, to my chagrin. He was just so happy that he leaped. He was fine. I didn’t expect him to do that at all. I’m carrying him up and down the steps.
Anyway, does anyone know why TJ may groan when he is lieing down? Perhaps it’s normal for some of them to make these groaning sounds. Would you be spending time and money, lots of money, at the vets trying to get to the source of the problem, like I am doing? Please comment as I will read every comment.