i hear water in my truck. why??

[moldman05]

I just got done changing out my temp gauge sending unit in my suburban. I drained what I could, put cap back on and did a quick unscrew and screwed in new one. I couldn't get all of the coolant back in the radiator so I put what was left in the resevoir tank. So there had to be some air left in the cooling system. The point of this is when I first drove off I could here a sloshing sound under the dash and it reminded me of your truck.