My 3500 knocks

This could be anything from a loose belt to a blown rod. You need to have a mechanic check it.

You may want to find a lightly used replacement engine and transmission. It sounds to me like you have a connecting rod bering going or gone.

either the valves or knock sensor most proberly the valves

you most likely spun a rod bering. knocking means that you have no oil pressure. most likely a bottom end rebuild is in order

The loud knock you here is a bering knock.(not good)By letting oil change go so long i would assume you have caused internal damage.And you say your oil pressure is 275 psi should be around 25 to 50 psi .Bering can't last long without oil.

check your fuel cap
use prmium gas

i would check the driveline, cairyer bering, and u-joints.

if it is a knocking sound coming from the top of the motor it could be something as simple as detonation this can be casueed by the incorect fuel grade being used or from years of carbon building up in the cyclenders casueing hot spots. if this infact knocking from this you could try some higher octane fuel or a additive to increase your octane rating and it should get quieter or disapear all together. this could also be cuased by a faulty knock sensor. if the sound is more of a metal on metal sound or farly loud you could be looking at major engein problems. such as a bering getting ready to go or a stuck or sticking valve. or a wrist pin that is getting ready to let loose. is the sound coming from the top of the motor or the bottom. can you here the sound louder on the left or right side of the engen compartment. also i need to know does the sound change with the speed of the motor like when you rev it up. also does the sound get louder or quieter when the truck is under a load like climbing a hill or hard accelerating? I will be able to give a more detailed answer if you answer thous questions also what grade of fuel do you use?

when you take the front wheels off you will see a silver center cap in the middle of the hub take a flat heat screw driver a gently pry the cap off (don't worry if you mame it alittle) there will be a cotter pin a nut and a washer then you outter bering take them all off then the rotor will come off the inner bering is still in the rotor being held by the grease seal pry the old grease seal off and remove the bering. check berings for wear like ruff spots. make sure you buy a new grease seal for the rotor. reverse removal procedure re pack berings with fresh grease inner first place bering in the rotor put seal in place will need to tap it in carefully when putting the nut back on tighten untill snug not tight check for specs if you want i usually make it snug then 1 more turn do not over tighten and you will be good

did you check the double cardan joint on the front driveshaft. if not lubed regularly the wear out and become sloppy