I have a Trane X78 furnace/A/C system that has begun to have a "tinging" noise coming from the blower motor. I took the covers off the front and watched the drum slow after it was shut off. The noise had a rythym to it that makes me believe that the bearings (if there are any?) may be going out on the unit. Could anyone provide a troubleshooting list for me to go through to narrow down or eliminate the source of the noise. The unit is 20 years old.

You might squirt oil where you think the bearing are ,although it would be temp. it might tell you something,if it is direct drive as most are, it will not help as the bearings are built in and part of the motor,, it also might be a broken fin in the blower cage itself