Did you get a new impellor shaft and rubber grommets too?
Normally the impellor shaft or rubber grommets wear and allows the impellor to move about more and create the noise.
Check the impellor shaft for any wear. They use to run a stainless steel shaft and it would wear out where the impellor spins but the top and bottom of the shaft remained normal thickness. If the shaft is not even all the way along its length, then get a new shaft.
The impellor shaft should be a tight fit in the rubber grommets. If the grommets are lose and fall off the shaft, they need replacing.
Impellors last for a while and to check them you hold the magnetic base with one hand and gently turn the plastic blades. The blades should only turn part way round. If you can turn the blades all the way round while holding the base, then the impellor is stuffed and needs replacing.
Little nicks in the impellor should not make any difference.
--------------------
Check the o-ring and make sure there is no air leaking into the pump, and make sure there are no restrictions, kinks or blockages I the hoses or fittings.