My gerbil (Frito) stomps or thumps his back feet what does this mean?

I have 2 gerbils in the cage. Soda and Frito, they are both boys and they have seemed pretty content. From last week until now Frito keeps stomping or thumping his back feet making a clear rythmn and noise. I can't see any pattern from the other gerbil from this or any socialising. But I'm worried is there anything wrong? Please help, will award Best answers, :)