You may have an air pocket in the cooling system. You need to run it, not drive it, with the radiator cap off and let it get up to temp, once the t stat opens the air pocket will escape through the open radiator. You may even have a bleeder screw on the top of the t-stat housing, if there is run it with the radiator cap on get it up to temp and CAREFULLY open the bleeder, I suggest wearing a pair of work gloves open it enough for the coolant to trickle out and the air if any will follow.