I have a 1996 Silverado with the 5.7 V8. There is a knock in the engine. It stops once when there is no load on the engine such as going down hills and when I get to high rpms. A couple of suggestions have been: Main engine barring, broken valve or valve spring and cracked flex plate.

Here is a link to a video of the truck knocking. You can really hear it about 50 seconds into the video.
http://www.youtube.com/watch?v=_Jla1jHRJ70

Does anyone have any thoughts?

 

before I tell you where im puttin my money. some questions...
whats the oil pressure at cold/warm-up and full warm-up?
did you check oil level at any time and after running check again to see if oil was consumed?
miles?
with this sound, definately not a flex plate issue. that would sound like marbles in an aluminum can.
Oil pressure would be affected dramatically due to this noise.
valve train components do not make the infamous "little man knocking on the door"... if it was a broken valve spring or valve, that engine would have a misfire code, and youd be hearing much more bad things than this. though my money goes to rod bearing noise...
My vote as to why it goes away at idle of when no load is present is because the oil pressure pushes enough oil to cushion the severe clearance in your bearings. it sounds like it may only be one rod bearing, but depending how much it was driven and how hard, that rod may need servicing. If I were you, i would not drive it more than absolutely needed. with that sound, your close to spinning that bearing and if that happens you really run the risk of not only trashing the rod, but the crank bearing journal. trust me, you can service all your bearings and crank while motor in frame. its a hell of a job, and only way is with the vehicle lifted all the way up a post lift. reason i asked if you notice oil consumption is since your bearing clearance is now larger, your rod will be flinging more oil onto the opposite piston bore/skirt and the extra oil will overload the oil scrapers and cause the oil to leak into the combustion chamber and burn off... little by little but it happens.
if you busted a spring, misfire code like no other for that cylinder. though if it was a busted valve, when that valve piece drops to a piston going up and down faster than we care to calculate, it will crack that aluminum piston and turn your motor into a very loud can of marbles... not to mention total compression loss and terrible idle and the fuel economy would just be down the sewer...
I will say i am sorry for this recent find of yours. it sucks... big time. wish it was just as easy as a flexplate. maybe someone else can prove me wrong because i hope i am so you dont get to worry about that mess...

 

Thanks for the reply. I've only had the truck for about a week. I got an amazing deal on it with the intention of putting a new engine in it, but I was just check around to see if it was a simpler fix. I haven't noticed any thing with the oil, but I've only driven it about 8 miles from the old owners house to mine.

BTW, I forgot to mention in the original post that the check engine light is on and the code is a cylinder 8 misfire. It could be just another problem with the motor, but I thought it worth mentioning.