fuel filter or air in line problem

thanks for the tips ..im going to take it to the shop this week...i dont think its anything major.. the reason i say that is i was getting any where from 6 to 6.7 mpg dep. on the work (dump truck) and today the truck was on e and i put 51gals in and checked my miles i got 315miles on that tank and have about 10 gals in the tank..so my fuel (mpg) is still in line if it was something major i would think that would suffer...also gonna get a muffler cause this rattle is running me crazy,and hope to make it a little less noisy with a little more pipe on the top..mike

 

i want to thank all of you guys for the input and help...i got the problem solved ... i counldnt take that rattle in the muffler so i stop and got 1 up it on and problem solved...all the inside of the muffler was broken lose and rattling so i guess the pop was the air being cut or blocked from the lose metal...old truck sure is quiet now and glad i didnt go with just the pipe ...i can here myself think now lol mike