Growling Noise In 2wd Ext Cab 1500

[05on20s]

So here are the symptoms. The truck has been sitting for only a week. Down here in MD it’s been pretty cold and we finally had so some. So I drove the truck to work today and for some reason I was hearing a growling that I could also feel in the truck. I heard it at really low speeds and then I heard it once again around 50 mph. I heard it one time going up hill for a slight sec then went away. I am thinking since the tranny was cold and the fluid was trying to warm up that might be a cause but I wanted to run it past the truck gurus. Fluid in the tranny is clean and the levels are perfect. I planned to get a shift kit installed and a few other things serviced along with new filter and fluid next month. Truck does drive perfectly normal though. It’s a 1500 ext cab 2WD w/ drums. Any ideas?