My furnace started making noises in the last few weeks. Today it got a lot worse. The noise occurs only when the blower is running and it sounds like a high frequency vibration. I removed the blower assembly and the fan/motor turn freely and it does not appear that the bearings are grinding. When the motor shuts off it takes at least 30 seconds for the fan to coast to a stop. With the fan running at top speed, when I release the door safety switch (killing power to the motor) the noise stops immediately, even though the fan is still turning at a high rate of speed. Does this sound like a bad motor or do you think it's something else?

It could be the cage wheel. The item that spins on the end of the motor and also make sure the set screw(s) are tight. They can go out of balance. Also check the motor for shaft end play and that the motor and band mounts are tight.

Thanks for the feedback HAC1000. I check the motor mounts and they're all tight. The shaft does not have any play left or right but the shaft does have approximately an 1/8" of play in and out. Is that normal? I think you might be right on with the cage being out of balance. Is there a way to rebalance them?

Is there a way to rebalance them?

Not really they are a replaceable part. Usually one of the little balance clips fly off and then the noise starts. 1/8th play is not bad at all.