The impeller or the impeller cover is shot..
Same thing happened to my mag 9.5
I bought a new pump but also a replacement impeller and impeller cover and o-ring...
They wear out over time...
Contact coralife, dont tell them it was used when u bought it as new....
Tell them the situation, explain you tried cleaning with water and vinegar solution and see if they will replace the pump for you
There website is
Energy Savers Unlimited | Coralife | ESU Reptile | Birdlife | Pondlife | Deep Ocean | A Worldwide Manufacturer of quality pet products
you can find how to contact them via phoen or email there..
Hope its works out for ya, let us know how you made out...