Thumping problem

I have 5 baritones ukuleles that I play regularly. On 2 of them, when I strum I have a tendency to thump the soundboard. It's only a problem on those 2 baritones.

I have 3 other baritones that I never thump. The 2 thumper ukes have identical-looking structure to the 3 non-thumper ukes so I can't figure why my regular strum makes a thumping noise on just those 2.

Any suggestions or should I just sell the 2 thumpers?

Check out the string height & the body depth on them, maybe a bit different.

Maybe the thumping is cause by a slight variation in your playing technique?

I know nobody wants to admit such things, but if you slow down how you play, and carefully observe, maybe it's not the ukes in question, but instead how you are playing them?

Can you have a friend play the instruments, and see if the thumping is present when someone else plays them?

This could help identify the cause of the thumping, of player vs. instrument...

I have the same problem with my Eastman Concert which, incidentally, has the lowest string height of all my ukuleles.

Yes, I think the action is too low on those 2 ukes. Thanks for the help, you folks!