My 4.9 in my Deville recently started making noise after a cold start. It starts making the noise about 30 seconds after start up and goes away within 1-2mins depending how cold it is outside it seems.
I noticed it start doing this after doing an oil change recently, but it has also started getting colder out around the same time as the oil change.
Its almost like a knock but a bit higher pitched than a bottom end noise. It doesnt have the bottom end rattle right on fire up just this noise after as said above. Its not a lifter tick.
After the initial minute or two it doesnt come back until it has been parked long enough to get cold again. Warm starts its quiet and I never hear any noise from it at any rpm while driving or loading it up in gear with my foot on the brake.
Oil is Castrol 10w30 and filter is a Wix. A couple days after the change I added Lucas oil addative to it to see if it would help, but still the same. It was a bit louder this morning after sitting for a few days and it being cooler and wet this week.
It does increase with engine rpm, but I dont think it gets any louder. Its most noticable inside the car with the door open. Harder to hear standing at the front of the car.
Is there a common issue this could be?

 

I have had a couple 4.5's with similar noise. Still driving one now. My guess is that it is piston slap if you have a lot of miles on the car. Not very noticeable when its warm out or motor warmed up and not accelerating hard. I got a couple spare motors because I thought first car would give up engine, but it didn't. Put about 25k miles on it and sold it at about 180k miles, still running last I heard. Liked he car so much I sought out another black Coupe DeVille to use as winter car. Have put about 60k miles on that so far, but think it is developing same noise at about 181k miles now. Still have my two spare motors though, so continuing to drive, keep oil and fluids up and keep an eye out for a lower mile motor. Long as you have an extra motor, they tend to keep plugging along. If the additive was a thickener, that SB ok, but if a lifter treatment I'd change oil again and put as thick an oil as ny climate will allow car to start ok.