and if you spin it while its off, is there resistance/rubbing?
I don't know about your ehiem, but with Any Hagan products, there is a ceramic "shaft" that the magnet/impeller rotates around. It'll technically run without it, (sometimes) but it rattles around and is very weak - Kind of like the hesitation you are talkinga bout.
I think this is what it looks like in you filter?:
(Correct me if i'm wrong)
Its the bit that runs inside all that, its kind of like the Axle. If its broken, that could be the cause of your problems/noise.
They're pretty brittle, so its easy to crack them with bending if it wasn't aligned when you put it all back together.
Also, after a good cleaning, my Fluval is always a bit louder. Not as much slime lubercating/insultating the pump
Hope this helps!