My 99 Durango has been changing its tune as of late. Not sure how to describe it, but normally its a nice and quiet, normal Durango. Sometimes, usually for a day or two, it'll be loud, then for a few days its quiet again.

The sound is kind of a drier sound, like a loud fan. It struggles to hit the higher RPMs, usually capping about 2k, and accelerates a little slower. Mileage seems to be the same, if not its like 1 mpg lower in its loud state. However, nothing was out of the ordinary during my checkup (currently it has a small rear differential leak and a decent oil leak at the rear main seal). Id like to know what might cause it and, of course, if i need to get it looked at or fixed. It did it with a full tank and with an eighth of a tank, and speed doesnt seem to matter.
Tires are brand new, brakes are in good shape, all parts are stock and the vehicle has never been in an accident; has just under 165k miles on it.

1999 Dodge Durango
5.9 V8
4WD

 

Fan clutch failing? That would make some noise, but, while it may make the engine feel a bit sluggish, it wouldn't limit RPM like you describe. That sounds kinda like a cat that's clogging up.