Dunking sound from heater unit ´94 XLT explorer

[mike_versluys]

After driving for a while, engine at working temp. when we hold or stop, you can hear (feel on floor in front of passenger seat) a loud clunging or knocking, like having in a metal tank half full of water rocking around. It looks to me as having air inside cooling unit, and sweeps around causing this noise to come. When engine revs high, there is no sound at all (apparently)
This sound apeard after having replaced cyl.head , as there was a crack between in- and outlet valve port holes.
Thank you in advance.

 

[MrShorty]

That could just be air in your cooling system, but it should flush itself out, too. Is the radiator full?

 

[mike_versluys]

Dr. Mr. Shorty; Radiator is full, but I do noyt know if there is a special way to bleed the heater unit, or if the small pinhole in thermostat is located the wrong way. Thermostat was replaced when cyl. head was replaced.