Thump beneath floor pan

I'm a bit stumped. A few days ago I removed the floor pan and gearbox tunnel to access the diff lock switch (whoever decided that the switch must be placed in the most inacessible place ever deserves a public stoning, in fact I gave up and left it be) and replaced the panels (minus 3 or 4 which got lost, rounded off, broke the captive plates, etc). Since then, I have started to feel a thump beneath my feet whenever I drive over any sort of undulation (loads of them where I come from). At first I blamed the floor pan. Then I remembered that I had observed a deepish split in the offside gearbox rubber mount so I replaced that last night. The thump has diminished but is still fairly noticeable. Could it be that 2 or 3 missing screws in the floor pan are causing it?

A guess i would say engine / gearbox mounts or maybe a loose exhaust or something, is the battery strapped in could have come loose

Found the culprit. It was a missing nut on the nearside gearbox rubber mount. All's silent now