Clunky sound on lathe

I've got the MC900 type lathe from Hare and Forbes and there is a clunky sound when I come down in speed. So, what happens is that the lathe runs fine at all speeds particularly the high speeds and then when I reduce the speed down to 800 or less there is a rattling sound. Belt looks ok, pulleys look ok and I have lubed the machine. I have not changed bearings but would faulty bearings work well at speeds above 800 rpm. If it is bearings, what bearings should I buy and what size, manual that came with machine lacks that information?