we have had a few customers bring in dyson dc25's with noisy brush rolls.
Upon closer inspection we have found that the cam that drives the brush roll is worn and damaged.
In every case I have dealt with so far the customers have admitted to using the machine on there laminated wood floors( very big in SA) Without switching off the brush roll.
this causes the brush roller to spin faster as there is no resistance like their would be on carpets and the cam fails.
So if you have a dc25 switch off the brush roll when vacuuming your hard flooring