Thank you for your quick reply, Freon!
No, it's not a direct drive ... I changed the belt recently and it was okay for a while. If the furnace has been off for a time, as after the summer, or even after turning it really low for a half-day or so, it won't make that noise for weeks, sometimes over a month. Then, all of a sudden, it will start doing it again. That's when I turn the heating off for several hours and give it a rest again. I usually oil the motor bearings or check the belt and see if it's too tight or loose. I had the furnace off for several hours after I posted and now it's back on again and not squeaking.
I was wondering if perhaps something like this could happen if the air in the basement gets too hot and dry that it brings on the squeaking noise in the wheel or dries up the oil in the bearing. In my other thread I mentioned how there was an extra hot air duct opening down the back of the furnace unit. The basement is easily the hottest room in the whole house because of it. (please see other thread)
But I will see how long it goes for this time without making noise. Then I will change the belt again and maybe clean any dirt or grease from the grooves in the wheels. (Or try to, the unit doesn't conveniently open from the side to allow access to the back wheel and blower. It opens from the front and there is no way to get to the side because the water heater is there).