What would cause my 1993 silverado 5.0 133K miles, to knock for a few seconds when started. Sounds like the back of the engine area.
The noise you hear might be piston slap. Not uncommon on these engines.
Piston slap occurs when the skirt of the piston slaps against cylinder bore as the piston moves up the cylinder. Then as the cylinder and piston heats up the piston fits tighter into the cylinder and the slapping stops.
Piston slap is not damaging. These engines can go forever with piston slap. It’s just embarrasing to have an engine make this noise when it’s first started.
The piston slap that I have experienced did not go away in a few seconds. Instead, it took a minute or two until the piston warmed up, expanding to tighten up its fit in the cylinder. It may be a valve lifter, also a harmless situation if the noise goes away every time.
It could also be a slightly loose rod bearing.
To the OP: Does it happen at every start or just once in a while?
It happens 90 percent of the starts.