I'm with Bobby-M. The magnet on the motor shaft should free spin very easily with the housing off. It being "tight and ruff" sure sounds like the bearings are the issue.
First I'd check the fan blades to make sure none are hitting the housing. Chugger has a video show how to do this with a screw driver.
Here's where I warn you that I'm not afraid to take things apart and trouble shoot. If you are not comfortable doing this skip to the next paragraph. Second, I'd hit the local auto parts store and get electrical connector cleaner and some 3-1 oil. I'd pull the housing off the motor. To get a better look at the bearings. I'd clean and oil the bearings. I'd inspect any contacts for corrosion and clean as needed. I'd reassemble and rotate the shaft by hand feeling if there was any improvement.
Or you can just oil the bearings through the oil hole rotating the shaft by hand. And, hopefully you will feel the bearings free up and the shaft will free spin.