Pull off the pump and clean it out.
The noise could be sediment build up causing pump rotor to make noise.
Noise could also be bad bearing or bad rotor.
Noise could be air bubbles in the line.
Once pump motor begins to slow, no mater what the cause, it overheats and begins to burn out.
Many times, once a motor has slowed, it never recovers and spirals downhill and stops working.
There are no repair parts for recirculation pumps.
Following page has general information plus links to Grundfos manuals, installation, and contact.
http://waterheatertimer.org/Water-heater-recirculation-system.html#GrundfosAdd a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.
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