I has a Rheem RGDA-150C-JR (made in early 80's). It works fine until... this afternoon I notice a lot of noise. I check the blower assembly and found the motor was very hot (but not stopped running). The assembly has two wheels with the motor installed in the middle (double shaft). I saw that one wheel ran steady and smooth, but the other vibrated a lot (up to 1/4 in). I check the shaft with my hand after the motor stopped, but I didn't feel much side play. Not sure if the bearing was bad. I saw there are balance weight (a small of metal) on a blade, so I suspected whether such a piece was falled off the wheel blade causing off balance vibration...
I believe motor over heat was caused by the vibration that over loaded the motor. Then the question is how do determine what is wrong -- a bad bearing or a bad blade. As I mentioned earlier, it seems the problem came from one end of the blower assembly. So I would assume the run capacitor is good.

THANKS!

Check the blower wheel for any dirt or paper that might throw it off balance, if it is dirty disassemble and wash. If these wheels get out of balance generally you need to buy a new wheel.