2005 GM Ultimate bot at 40k on election day 2008 and now has 75k

Lately having an engine knock issue. After the car sits for 8 hours or so then started up I'm getting a second or two lower engine knock which quickly goes away. Like the knock you may briefly hear after an oil & filter change.

Changed the oil and filter last weekend - same result.

Any ideas / suggestions will be greatly appreciated, tkx.

 

I am having the same issue on my 2011 GM.

 

The problem you describe often pops up after an Oil / Filter Change -- for 2 reasons:
1. - New oil is NOT of factory-recommended viscosity.
2. - New oil filter does NOT have a Silicone Anti-Drainback Valve.
The fact that 8 hours between starts brings the problem indicates that the oil is draining back out of the oil filter,
leaving a "dry start" condition. Most quality oil filters have a silicone anti-drainback valve, but many do not have that important feature.
'92 thru '98 FOMOCO 4.6-liter engines require 5-W-30 viscosity --
The '95 MERC gets 5-W-30, Full Synthetic, SN+, ILSAC GF-5 Oil -- and a Bosch #3410 Oil Filter.
This combination of oil and filter gives the best all-around performance for the lowest cost and nationwide availability.
If at all possible, DO IT YOURSELF !! The LUBE SHOPS use cheap parts and cheap-*** oils, and they are NOTORIOUS for not refilling the crank-case.
A damned DEALER forgot to fill the wife's conversion van, but the kid at the gas station across the street checked the oil, and found it was EMPTY.
I'm 80 years old, but I still do almost all of the work on the 95 MERC -- it's fun, and the money I save pays for vacation trips !!