What is the deal with my truck?

[Jackson]

I have a April 2004 F250 6.0 with 198k miles on it. Being a 6.0 it has been quite problematic for me. 2k miles ago my fuel pump died taking my injectors with it and all of them had to be replaced. While it was in the shop, my mechanic noted my head gaskets/headbolts were in fantastic shape having been replaced shortly before I bought my truck (I had no knowledge of this).

Anyway I'm still having issues and I need help on determining any possible causes before I take it in to the shop.

1. When I'm at idle I hear a shuddering vibration sound coming from my dash (driver side louder than passenger)

2. When I first start my truck after sitting for a few hours I hear a squeaking noise coming from the front of it and can even hear it from 70 feet away.

3. One day earlier this month my fuel economy suddenly decreased. I was getting an avg 16-17 hwy and now I'm lucky to get 10. My Scangauge is reading 48v FICM and my injectors are practically brand new and it's about like it was before they were replaced.

If anyone can provide me with information about what could be causing these issues I need it ASAP!

Thank you!

 

[Gary]

Well first its hard to dianose a noise from desription but I'll thow out a couple guesses.
On #1 maybe the up pipes leaking or down pipe rubbing on cowl
On #2 something in the belt drive
On #3 Is the trans going into OD? fuel leak? any codes?

 

[sootie]

belt tensioner is first guess

 

[Zmann]

I can't imagine a visual would tell you if the headgaskets and bolts were new ?
at least not in My state where we don't have salt
my headbolts looked new when they came out for studs