Piston slap or lifter tap???
GM Service Manual Info regarding "Engine Noise on Startup, but last only few seconds":
A cold piston knock which disappears in 1.5 minutes should be considered acceptable. A cold engine knock usually disappears when the specific cylinder’s secondary ignition circuit is grounded out during diagnosis.
A light rattle/tapping noise may indicate a valve train, upper engine concern, or a low rumble/knocking may indicate a crankshaft or piston, lower engine concern.
Incorrect oil filter without anti-drain back feature – Install correct oil filter.
Incorrect oil viscosity – drain the oil and replace with correct viscosity oil.
High lifter leak down rate – replace the lifters as required.
Worn crankshaft thrust bearing – Check the crankshaft end play, inspect thrust bearing and crankshaft, repair or replace as required.
Damaged or faulty oil filter by-pass valve – Inspect the oil filter by-pass valve for proper operation, repair or replace as required.
You have already eliminated the last item.
A cold piston knock which disappears in 1.5 minutes should be considered acceptable. A cold engine knock usually disappears when the specific cylinder’s secondary ignition circuit is grounded out during diagnosis.
A light rattle/tapping noise may indicate a valve train, upper engine concern, or a low rumble/knocking may indicate a crankshaft or piston, lower engine concern.
Incorrect oil filter without anti-drain back feature – Install correct oil filter.
Incorrect oil viscosity – drain the oil and replace with correct viscosity oil.
High lifter leak down rate – replace the lifters as required.
Worn crankshaft thrust bearing – Check the crankshaft end play, inspect thrust bearing and crankshaft, repair or replace as required.
Damaged or faulty oil filter by-pass valve – Inspect the oil filter by-pass valve for proper operation, repair or replace as required.
You have already eliminated the last item.
