zachsf150

i have an 2008 ford f150 4.2 v6 and when i first fire it up when its cold the fan clutch stays engaged and when you give it gas the truck gets louder as if it has an exaust then doesnt really back fire but it makes a different noise when i let off the gas. has anyone else had this problem, and any ideas on what might be wrong? once the truck is at operating temp it doesnt get loud and the fan clutch doesnt enage. also the truck has been getting very poor gas mileage if it might have anything to do with this problem. this has been bugging me for a few months because the truck is stock ands only has 30000 miles on it!