Has anyone running the 4L60 4 speed automatic tranny... every hear a "slap" sound when you first fire the truck up and start driving.

On my 09 chevy pickup, it does this sometimes but it'* only right when I start it up and only with it'* first shift from 1st to 2nd and it'll do it whether it'* cold, or even if it'* fully warmed up and I stop somewhere and come back out and drive off... it'll do it again.

It doesn't seem to hurt anything and the shifting is normal... I was just curious if anyone has run across this and knew what it was and if anything could be / should be done about it.

Had something similar on my 1995 Roadmaster. Start it up and an almost inaudible thumb and on very level ground, and the car would move just a tad then bounce very slightly off of the park pawl.

Almost like a clutch pack is slightly engaged while something bleeds down or something needs to pressurize to release. Not sure if the exact same cause etc. but sounds like they are distant relatives (even if they are 14 years apart).

Filter/fluid swap and new shift solenoids didn't fix it. Worse when cold but still just something I notice, not something I worry about.

Yessir, I've been noticing it since I got the vehicle at 6,450 and now I have about 18k on it and she'* still shifting / performing just fine.

I figue on doing what you described... not worrying about it.

Sounds like a plan.

If it was under warranty and easy to display I'd take it to a dealer, but if out of warranty like mine I'd just live with it.

FYI while I'm thinking about it, I tightened the bolts on the valve body while I was in there. Just a little. Didn't help this issue.