Assuming you've done the obvious - checked for blockages and changed the filter - then it's almost certainly crud stuck to the vanes of the impellor, at least that's usually what happens to my workshop vac.
In mine (Earlex) it's hard to get at, but, by sticking the motor in a plastic bag in the freezer for a couple of hours, the plastics shrink enough to pop the pressed steel cover off the fan, to get at the impellor, which I can then gently scrape clean with a flat bladed screwdriver.
The other possibility would be the motor bearings. That happened to our household vac, when our cleaner thought it was a wet+dry (it wasn't :-( ). I dried it out, but the bearings screamed whe it was started again afterwards, and soon the motor failed spectacularly.
If it's the former, I've managed to keep the Earlex going for about 15 years with occasional reaming-out. The household one that died (a "Gnat" - a spin off company from Numatic, lots of common parts), only managed 27 years - rubbish really.
Hope that helps...
E.
