Two possibilities:

1. Motor bearings need oil -- 10W30 motor oil will work. Use a pump
oiler with a long spout to reach:

a. Bearings (2) on motor
b. Bearings (2) on shaft ends of fan, if belt driven..

You may have to loosen things to pull the fan assembly out so you can
get at it.

2. Slipping fan belt, if so equipped. There should be a tensioning
screw or other adjustment. Set so firm thumb pressure depresses belt
about 1/2" It also sometimes helps to use "belt dressing" on the
belt.

I keep a spare belt on hand at all times. Size is stamped on the top
(outside) of the belt. Any good hardware or auto parts store.

Check belts and/or bearing twice a year. Check burning/pilot light at
same time. I check at the beginning and the end of the heating season.

Do filters monthly. Consider a "washable" filter and spray-filter oil
for better dust pickup.