[azeotrope]

I followed your advice. At first, it was very difficult to spin it manually, as if there is resistance. Once it pick up speed, it can spin by itself. Now, I am happy to say it can spin by itself. There is no longer any resistance when I spin it.
BUT now there is a nasty grinding noise when I turn it off as seen in this video I made for you

I can live with the nose at the end. but hope it does not spin itself to death in a fireball blaze

I’m guessing your bushings have failed. Time for a new motor.

 

[azeotrope]

With all the searching and researching, my experience tells me you should buy a brand new exhaust fan as they are an inexpensive and easy to replace. Sorry if it seems blunt but it's true.
Good luck

As a tech who replaces them in the field often, I can confirm that. Sometimes the new fan actually end up being cheeper than the blower assembly if it were purchased separately.

You will save a lot of labour if you can find a new fan that uses the same blower assembly as your existing. Pull the guts out of the new fan and replace the internals of the old fan. No need to re wire, or mount the housing, or reconnect the ducting, or save and clean the blower wheel. Install the new motor assembly and grill then it is good to go, about one screw and a plug worth of work.

Manufacturer’s often use the same blower assembly across multiple of their fans in their product line. May need to contact the manufacture for that info. They are typically happy to help.