It's been very cold lately in the NE

I have a 04 TL auto with 98k miles. Recently when it's cold outside, the engine gets very noisy even after the car warms up. It's not a clicking or grinding noise but rather a heavy tone. The engine revs @ around 1800-2000 rpm cruising 60mph on the highway @ 5th gear. I tried the manual shift and only used the 4th gear on the highway (2200-2500 rmp) and the engine sounds much better.

Is it ok to use only the 4th gear on the highway? will it do any damage to the tranny or engine? and has anyone experience noisy engine when it's below freezing point?

Thanks

 

got video?