Did you notice any reduction in flow? If it's properly primed and running quiet at first, somehow air got in there a while later as described, you may want to check the intake hose/prefilter/adapters(quick-disconnect, L-bend near the body) making sure nothing is clogged. A clogged intake could cause cavitation and noise. I have never seen this in my 2217, but my 2213 (12/16mm hose) would do that reminding me of cleaning the intake/outflow hose.
I even replaced both the bushings and shaft. It was quiet for the first few minutes, but now its noisy again :/ not nearly as bad as before, but not as silent as when I first got it, either.