Hi all, I have an 03 Silverado, 5.3l V8 Gas engine. On cold startup, after sitting overnight or longer, it makes a horrendous slapping sort of noise. My immediate assumption was that it is a piston slap, but it sounds so awful sometimes I wonder if it's something worse, and idk what a piston slap sounds like. The engine takes a lot of coolant, maybe a gallon a year, I've changed the plugs and there was no white/sign of coolant in the cylinders. Also had a hydrocarbon test done and returned no results of coolant in the oil. Any ideas? Below is a video link to the noise, in the video it is at a medium-volume compared to what I've heard in the past. It can be double the noise and intensity at times.

 

Sounds like the normal piston slap on my 01. Not sure about the coolant issue. Mine leaked from a bad radiator; replaced that. Also leaked from a bad water pump; replaced that. Check both those things. Just let the slap-sound go away (oil getting throughout the motor) before you rev it and drive.

 

Sounds like piston slap to me.

My 00 5.3 does that when it's colder. I've put remote start on it so I typically start it 1 or 2 min before I even get to the truck.