If you don't find anything rubbing on the blower then it's probably the motor brushes. As they wear the brush wire runs out of slack and they start loosing contact with the amature. It's a very easy task to take the motor apart once you get the assembly out. Remove the connector adapter and the one screw on the end of the housing. Pull the motor out and with a little bit of solder iron work you can creat some additional brush wire slack and improve the contact. Also oil the bushings while you're at it.