Mylo,
What brand/model furnace?
At least on the Atwood 8500 series furnaces the fan motors use non replaceable bearings ... bronze maybe? While they are generally considered throw away motors, others have been able to re-oil the bearings.
While the jury is still out, I have re-lubed 2 motors so far. As a matter of fact I will be changing one of those out today as it is making some noise on start up like yours and possibly running a little slow. The one (call it #2) I will be putting back in I consider a better test of this method as I did a better job of re-oiling.
When I took #2 out it was hard to turn using the squirrel cage fan ... could not turn by hand with the fan removed without using a wrench. Once removed gave it a couple of shot of WD40 (I know, not a lube) just to loosen it up. Then over at least a couple of weeks, with the motor sitting vertically in a hole punched in a paper cup, would add 2 or 3 drops of light machine oil (3 in one oil) to soak down the shaft. Next time by, would turn motor over and lube the other end. Did this sporadically for at least a week or two.
When done it turned freely by hand. It is the one I will be putting back in today .... will try to give an initial update afterwards. Real test is how long it lasts ... but beats $50 and up for a replacement motor so far. The last swap was sometime last year ... should have a note somewhere but haven't looked yet.
I did take a motor to my favorite generator rebuild shop and he said nothing to replace the bearings with FWIW ... we use our RV year round at different times so do put some hours on the furnaces. Currently in Mojave, CA for a while and it has been getting down to around freezing give or take at night.
So as I said, jury is still out here, but it is doable if you want to go that route. Change out on the Atwood is from outside and not too bad.
Howard