I dont know anything specific about the Ninja but the the problem , noise and lcak of movement etc as described would make me think that the capacitor(s) switching is not getting in to run mode.
Its quite a simple repair , just a litttle more difficult than new brushes.
The capacitor will have its details printed on it ---type, capacitance,rating this last is a number followed by u or uf. Low valuse are sued to suppress interferenc from electric psiking larger values give a high start current and then an evened out one for running.
The hardest part is finding the right capacitor
Sometimes ie in old electrci organs some capcitors are so big that they have to be discharged for safety , but this should be need in more modern stuff.
Hope this helps