I got a pair of 6100's used from someone & when I saw them up & running in his tank, they made a clicking sound when they alternated. I later found after I disassembled them that the breaks & impeller collar were broken & the collar was rubbing on the impeller shaft wearing it down in a half moon shape at the point of contact. I had to buy a rebuild kit to replace these broken parts. Not too expensive.
BTW, the 6100's were only 3 months old but Tunze advised the parts were not under warantee as the likely cause was the high +500 PPM calcium level in which the pumps were subjected to.
This precaution is found in the product literature & should be followed to extend the life of the stream.
HTH as a possible hunt for the problem.