Agree with Sky-O - could be different things. One of my fosters thumped in our spare room to tell me she'd dropped her willow ball into her water bowl and couldn't drink bless her. I also get thumps from Truffle everytime I put her down after grooming her and, another bun thumps when I go outside as if to say "she's coming, food, food!". Well that's interperation anyway!!
If there's something scary in the garden I also get thumps but they're what I'd call proper alarm thumps. They all seem to be able to distinguish between them oddly enough as the first three don't lead to having lots of startled rabbits, but the last one does, unless they all saw the dnager. Oh shut up Caroline.....