Noisy roof air conditioner on Monaco Dynasty 1998

We recently have noticed an irritating cavitating-type noise in our DueTherm (Dometic) 620315 Penguin roof air. It works great but it seems like whatever is whirling around to force the air out, needs lubrication on the ends. Its not squealing, just a sound like its uneven as it whirls around. Doesn't seem smooth like the front unit. We recently had the insulation on the roof units replaced and I thought the four attach bolts were loose so I tightened them down. Still making the noise. Any thoughts from the experts out there would be appreciated. Can something like this be lubricated or fixed, or is the unit sealed and must be replaced? We have a 1998 and regularly run the fan all night long on this unit so its had a lot of use.
Thanks! Actual model nuber is 620315321;