My Accord was overheating so I replaced thermostat. After the repair I let the car idle for approx. 20 minutes. Put it in gear , clanking noise. I realized I hadn't topped off coolant so did that, let the car cool down and seemed to be fine. Then it did it again but only after idling for a while. Changed fluid with honda at. Still having the same problem. Once I'm moving it seems to work fine. Any help would be much appreciated

 

You should be able to top off the coolant before the engine is ever started.
Turn the heat to full hot(opens heater core valve).
Open the bleeder screw on the back side of the thermostat housing. Attach clear hose to
Car should be level.
Fill radiator til the bleed valve has a steady stream of coolant.
Close bleeder valve.
Fill radiator.
Dump old coolant out of reservoir, refill with fresh to cold fill line.
Start up car and let it fully warm up(fans turn on).

As for the clanking noise, no idea.
Only time I had any kind of clanking noise was when the axle broke, but the car wouldn't move.