We have ~ 1991 Carrier 58sx high efficiency condensing furnace and central AC. Now the air handler is making a loud humming/vibration noise when the fan kicks in (the blower motor for the air handler makes the noise, not the gas furnace/exhaust blower). The serious electrical humm is loudest when the motor is just cranking up, but remains pretty loud now when it's blowing at full rpm (vibrating noise too), making it hard to sleep at night (Utility closet is in the bedroom). What is the most likely reason the motor is now making so much noise -- when it seemed to be very quiet just a coupe of years ago? Bad motor? Bearings in the air handler (squirrel cage)? Needs a new motor? Suggestions? Thank you!

Could be dirt on the blower wheel that is attached to the blower motor. Could be bad capacitor on the motor. Could be bad motor.

We have ~ 1991 Carrier 58sx high efficiency condensing furnace and central AC. Now the air handler is making a loud humming/vibration noise when the fan kicks in (the blower motor for the air handler makes the noise, not the gas furnace/exhaust blower). The serious electrical humm is loudest when the motor is just cranking up, but remains pretty loud now when it's blowing at full rpm (vibrating noise too), making it hard to sleep at night (Utility closet is in the bedroom). What is the most likely reason the motor is now making so much noise -- when it seemed to be very quiet just a coupe of years ago?? Bad motor? bearings in the air handler (squirrel cage)? Needs a new motor? Suggestions? Thank you!
Thanks for your help! I like the sequence of your suggestions too -- simple and easiest/least expensive solutions are first! Looks like the pan may have been leaking at one time before we moved in, so I may find clumps of dust attached to the wheel. Probably worth changing the capcitor while I'm in there as well...
Cheers!