Dodge Durango 2003 SLT 5.9 engine is humming to a loud noise during low rpm (low gear)?

I have a Dodge Durango 2003 SLT 5.9 that runs perfect except during the last few days. Upon start the engine emits a noisy sound at low rpm low gear and will have its normal low humming sound once shifted to the 3rd gear and above. I have noticed as well that after a long constant speed drive of 50-55mph again (at least an hour) the engine will have the same noise when you slow down and start to accelerate again. Any suggestion/comment?