Workinonit,
A slightly bent valve would probably 'hang' open or closed...it takes very little to do this and it wouldn't make any mechanical noise. Piston rings wouldn't make a 'knock' either as they have very little mass and are always under some type of tension.
Properly describing the sound and when it actually occurs is crucial to troubleshoot the problem. When the truck is in fourth gear being 'lugged' you are loading all the engine components to the max. A crankshaft with excessive clearances with start to pound.
If the timing is too far advanced, the engine will preignite and ping. This too happens under load. The sounds don't resemble each other. Keep us posted.
Stuart
