HISTORY:
My '91 F250 ExtCab 4x4 Lariat (460cid) with ~200k miles that does not use oil or water, gets decent milage for a 7.5L (9.5mpg avg), and really no issues other than what is to be expected on a beast like this that is approaching 20 years of age. Motor was rebuilt around 2000, and I bought the truck back in October 2009, at which time the only problem I had (and still have) is the cross-filling issue from one gas tank to the other (I probably need to replace the rear 18.2gal tank's "Fuel Delivery Module" as Ford calls it). So with all that said...

HERE'S THE MAIN PROBLEM I AM EXPERIENCING:
Engine area is making a knocking noise that seems to be emanating from the lower end. It does it while idling in Park and also while driving, and the knocking does seem increase in frequency with increase in RPM's. I got under the truck while idling and this knocking noise seems to be originating from around the dual-pulley that both of the drive belts share. This dual-pulley is mounted to a shaft that appears to run into the lower end of my 460... can someone please tell me what this "shaft" is, is it the crankshaft?? Could I be hearing a crank bearing? My guess was a drive-belt pulley bearing at first, but after searching/reading several similar posts in the forum, I am now very scared to think that it could be a craft bearing. I recently replaced both drive belts (2 weeks ago) and thought maybe I tightened one too much and caused this condition, but could just be a coincidence. The noise started about a week after replacing the belts (last week). I changed the oil about 2000 miles ago and used a semi-synthetic 10w-30 (wallymart brand). also used a Fram oil filter, which alot of people here are saying do not use!

Thanks for any advice that anyone can offer, as this is my daily driver right now! Oh and sorry I am so long winded, I just try to include as much info up front as possible...

 

the shaft is your crankshaft, as far as it being crank bearings, possible, could be rod bearings also... im no expert on what im about to say just repeating what i have read on here, if you run conventional oil, you should not switch to sythetic what-so-ever (as i said, im no expert, just a parrot repeating lol)... as far as it goes from the way you describe it it does sound like it your bottom end,,, what i have done before is with an engine idleing use a stethescope on the oil pan in a couple inch intervals, normally if its bottom end you can hear it and pinpoint the area a little better before deciding what to do

 

Ok, all points noted, and thank you for your input. I have some more questions though, based on your response:

I plan on changing the oil again in the next week (as soon as I can find an extra half hour of time) so is it ok to now go back to a conventional oil after having used the semi-synth for 2-3 months? Also, I bought some Lucas Oil Stabilizer that I planned on adding when I change the oil. I had read another post where someone claimed that it helped their knocking sound go away... does anyone have any opinions on this tactic?

I will get my hands on a stethoscope of perhaps just some tubing and poke around under there to get a better feel for exactly where the knock is coming from. If it is the crankshaft, does anyone know about how much $$$ the repairs would be? I would have to pay someone to do it, as my auto experience is limited to replacing gaskets, pumps, alternators, etc...

Thanks again WickedClown.

 

Also, could I have possibly overtightened a drive belt when I replaced them, thus causing the condition with the crankshaft? I would guess that it would be more difficult than that to damage, but was just wondering for future reference!