Oh dear, that's not good at all. The repair shop must have heard the noise, yet still sent it out as a completed job? That doesn't bode well, IMO.

I can't understand why it didn't go together correctly for you in the first place. With the brushes removed, the armature should be free spinning, so if it isn't, it must have fouled on something. The thing is, there's not much that it can foul on, other than a mis-aligned magnetic ring, speed controller or power wire, any or all of which should be apparent with the black top removed.

A mystery!

Ray