Knocking sound from radiant heat

I have hot water radiant heat installed under my first-level floor. It works fine, but I get a knocking sound from one end of my bedroom when the programmable thermostat turns on the system in the morning. I'm familiar with baseboard radiant systems clicking and pinging as they are heated and cool off, and I'm sure it's a similar expansion and contraction situation. But the knocking is pretty loud (at least at night) and wakes me up before I'm ready to wake up. The radiant tubes are plastic installed underneath the wood subfloor. Is there some type of lubricant I can apply where the tubing is held up under the subfloor? Is there an alternate way to attache the tubing to eliminate the knocking sound? Help!!

Probably a staple up system and when the pipe heats and expands a loop of it hits the floor. Find it and isolate it.