On my last brevet, I noticed (heard and felt) a thump in the drivetrain when under power. It seems timed with crankarm position so I guess it's crank related not wheel or chain. I took it to the co-op where the mechanic checked the BB bearings for play and it seemed solid. We couldn't get it to happen on the stand. He suggested the cartridge might be loose in the shell (it's a Shimano Hollowtech Octalink). Unfortunately they didn't have the right tool to fit the lockring.

Question is, is his explanation plausible? If yes I'm tempted to buy the right tool and donate it to the co-op.