air in the cooling system. no big deal. It'll work its way out the overflow tank eventually. Just make sure the overflow tank has coolant between the min max. Watch it because it will suck in fluid to replace the air that is purged.

I'm assuming it got low and sucked air in or you wouldn't be having this happen.

there may be a purge valve somewhere on the engine to get rid of air more quickly, but I don't have a GD manual to check.