Dodge: 2000 Dakota, extreme engine noise, muffler shop, clutch fan

Question
I have a 2000 Dakota V6 2WD that was running OK until 1 week ago. While highway driving the engine noise increased/got louder when accelerating. It's kind of a loud roar.. There were no warning lights, temp, oil temp, battery were all ok. Pulled off and noticed noise went away. Noise has not returned until last PM. All fluids were checked and were OK, but noise has not gone away. Called service and they said that  a clutch fan is installed, and that loud operation "happens" in cold weather (less than 20 degrees F). There are no unusual noises, no loss of power, no steering problems or other change in operation of truck/motor. All belts/hoses appear to be ok. Nothing appears to be detached. The engine noise has simply gotten louder. At idle, engine sound is normal, but noise increases under acceleration. Is this engine noise increase due to colder weather? I bought used w/ 50K miles in Sept. now have 60K miles with one oil change in between.
Noise has increased during highway driving both times, and engine noise/loudness  remains.

Answer
William,
My dad has one also and it does the same thing.Of course the truck is not driven now due to him passing away,but i know that he had to replace the idler pulley,it seized up and came apart.I say this not really knowing if it solved his problem but it would be something to check.Also a catalytic converter could cause a noise similar to this if it has broken,But it will usually cause some sort of power loss.I'd talk to a muffler shop and let them tell you if this could be the problem because mine came apart on my truck and made a noise similar to this.