Sounds like air in the system, like i7Baby said.
Sometimes trapped air can be a serious problem rock the machine side to side and back to front to discover which movement gets that air to move.
Remember air always collects in the highest points of the systems loop, the moving of the machine is to make the air relocate itself and the space the air was occupying fill with coolant.
Make sure the reservoir is capped and full at all times and keep an eye on it's level, to be 100% sure the pump has coolant to it, never ever run the pump dry!
You have to do whatever you need to do to get that trapped air out of that radiator and the systems coolant flowing and as the air comes out, the reservoir level drops so watch it.