Mine started doing the same at max spin. Hammering really badly. I took the lid off and put it on a cycle. The concrete weight at the top had worked a bit loose and started to erode against the bolts.
I made some rubber washers up out of bike inner-tube and tightened the whole lot down.
2 cycles later it seems OK and I'm about to push it back in it's slot.
Mark
July 2014