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's only right when I start it up and only with it's first shift from 1st to 2nd and it'll do it whether it's cold, or even if it's 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.