07 Tahoe - Coolant Reservoir empty while running

[AZSCTACO]

So when the Tahoe is cold, the coolant reservoir is at the correct level. I noticed the reservoir goes empty when the truck is running. Is this normal? I can't believe I've never noticed this before.

Trucks temperature is perfect and no low coolant lights on the dash.

 

[Sparky_69]

Maybe air trapped in coolant system.