Problems After Oil Change

[04sierra4x4]

i recently just changed my oil from my trip across canada 3600km because the truck got sluggish 1700km into the trip and gas milage went to sh*t. So i changed my oil using mobil 1 5w-30, and 1 L of lucas oil stabilizer, and a fram oil filter and after when i started my truck i was listening to it and when i got under the bumper you could hear a slight knock. Could this be serious the truck runs fine and it seems to go away sometimes if i hit the trottle slightly.

 

Help Please

 

 

P.S.

any tips on milage help? truck gets 450km to a tank 5.3, sb rc and 33 baja mtz.