I have a 99 GMC with the 5.3 LS Vortec motor. The Truck is Standard and has now done 320,000 Kilometers.
There seems like a Tappet stile rattle above 2000rpm under load, and its getting worse. The engine doesn't burn enough oil between changes to warrant topping up, and apart from the rattle its working fine....

I would change the cam followers etc hoping that's the cause but have read some posts that it may be piston slap how can I tell the difference ???

Help...

 

Piston slap usally goes away when it warms up. But If its still does it when warm I'm going to say you have an excessivly worn cylinder or a broken skirt. We had a duramax at the shop that did the same thing. so we did cyl. cut outs while driving it and diagnosed it down to # 5 cyl. Pulled the head and that cyl. had lots of slop in the piston compared to the other 3. So we replace the engine. never did check to see if it had a broken skirt on that one. Yet.