I Perchased the heater Raypak 406,000 BTU last year,from some reason its nosy,make a steady squiking noise(very annoying),and some times it stops working.
this happened a few months after.
other than that, the heater pools out nice heat .
It’s hard to say for sure but it could be the internal bypass (spring and/or shaft). We would recommend contacting Raypak directly. It may still be under warranty.