Have you had a cursurey (sp) look at the brushroll bearings? Whilst the brushroll does do a bannana sometimes I;ve had dry bearings make odd sounds.
If you pull one of the end caps off, then withdraw the centre spindle attached to the other end cap, you'll see a bearing either side. With the spindle, insert the very tip of it int oa bearing (so the end wont poke out the the other side too much) and lever the bearing out. Does it feel dry/rumbly?
If the bearing has a black palstic cap on both sides then pop these off using a tiny flatblade screwdriver/tip of a fine knife from the inner (smaller diameter) edge, clean the air out and pop some fresh grease (or oil) in. If their all metal then you can't do much I'm afraid.
May be cheaper than a new brushroll, or at least help you troubleshoot! If the oil/grease helps matters then either roll with them until it happens again or just change the bearings. Their pretty cheap, im sure someone could order you 2 in (ebay can be pants for things like bearings!)