Greetings.

I have a new WaterFurnace NDV049 connected to two vertical closed-loop wells. After the installation, the system made a thump-thump-thump sound (3 or 4 loud, rapid thumps) when shutting off after a heat cycle.

The installer first suspected air in the pipes. But after hearing the thumping, he said it sounded like something else. He tinkered with the system, and the sound went away, though he said he wasn't sure of the cause.

After the system ran quietly for several weeks and then sat idle for a while, the thumping is back. Any ideas what might be the cause?

Duct work would be my off the cuff guess.
Do we know what specifically the installer did to fix the noise before?
What are the dimensions of the return air duct to the heat pump?

I wonder if these are waterside or airside thumps.

Waterside thumps might be related to pressure changes causing improperly restrained plastic pipes whacking a wall.

Airside thumps might be sheet metal ducts flexing upon air pressure changes.

Try cycling system blower on and off independent of compressor, then same with circ pumps

Agree with Joe...clarify "tinker" that temporarily cleared problem.