It is quite possible that this is what is known as piston slap. Mine does it, and it is the same type of sound/symptom I have had in my last 2 Silverados. GM Vortec engines are notorious for this. Not sure if the 3.8 uses short-skirted pistons, but that is why the GM trucks do it. Maybe I'm way off, but it sounds the same, and happens under the same circumstances.

Oh, I'm sure mine is piston slap.
I know its not an exhaust leak!

This was my first oil change at 1500 miles. I have had it for 4 months. I put Valvoline 5w20 in it, maybe I'll try the synthetic. I like to change my oil every 2500-3000 miles. If I switch to synthetic I wil let it go to 5000-6000 miles.

just an update... my first oil change (in colorado) i put mobile 1 synthetic and nothing changed, the knock was still there. I changed my oil last week here in S. Louisiana with mobile 1 synth again and the knock is completely gone and has not returned even after sitting for 3 days.

Mine started startup knocking after the oil change.
0W-20 Mobill1 and a Mobil1 filter.
5W-20 or 0W-20 Mobil1 STOPPED piston slap in my wife's Expedition, but I always used a Motorcraft or PurOne filter.
I think the next oil change (if not before) will be a PurOne or Motorcraft filter.
I don't have much confidence in the Mobil1 filter, now that I know it is the same as WalMart house brand.

In the driveway my jeep is pointed slightly down hill, I back in.

I don't have the problem.

If you park pointed slightly up hill, back in, see that fixes it.

:mad: I have the same knock when cold; I think I might run Schaffers Supreme 7k or 9k. I would atleast know that the metal had some protection... But it does sound like the GM piston slap crap..... Mine didn't show up until around 5k.:sad2:

I work offshore and park my jeep for 2 weeks. It definately is something that needs fixing. I'll try a better filter next oil change and see if that helps. And I'll probably bring it to the dealership and explain whats going on to see if they've had any brought in for this problem.

knocking upon start up
 

i had this same issue it only happen if the jeep sat for a few days. on the why home one day it started when i was driving it. i brought to the dealer they tore the engine down to find out it had a weak oil pump that caused two cam barrings and a crank barring to fall out of there location .they put a new short block in to only get it back two days latter to have a cam gear fail and take out the improved oil pump timing chain and 4 valves. what a lemon.total time in the shop 45 days. what to do what to do:what?:

I NEVER had this issue in over 15K miles, until a couple of weeks ago when I changed to Mobile 1 5W20 synthetic oil and a Mobile filter. Now it clatters for the first second after start-up, but only after sitting for several hours. I can turn it on and immediately off, then back on again and it's good, so it seems to definitely be oil/filter related. I am going to try swapping out the filter with a stock Mopar one and see if that is the fix, otherwise it might be related to the oil. I did do one change last year myself and had no issues, used the recommended blend oil and a non Mopar filter.

My JK is a daily driver and does not sit for extended periods. I will post up anything that I come up with.

There's got to be dynamic issues here, cause I've used Mobil1 10/30, with Mobil1 filter on my 94XJ since 95 with no problems, and after many mods, as well as hard wheelin', I've had no problems. The pressure is still like day one and no leaks.
We purchased the 07JK for the wife in Dec.06, and at 3K miles I switched it over to Mobil1 5/20 with Mobil1 filter and have been doing the same every 3-4K miles. No leaks, No knocks at start-up, NO PROBLEMS.