beetle mania

we came across this group of dor beetles on a track in the conifer woods near Carthamartha. There was no obvious evidence of droppings and the beetles seemed to be producing the moist mud balls by themselves. Some beetles had paired off and were burrowing deeper. Very faint stridulations were audible. Is this common behaviour and what were they doing? I have posted these pictures on Wild about Britain where someone will definitely be able to provide an answer. I feel fairly sure they are mating but I have never seen a collection of these beetles like this before (a colony seems to be the correct collective noun).