Jviduya

I have a Max e therm 400 heater. Recently it stopped working. After diagnosing it, I replaced the gas valve. So it started firing up...but within seconds, I started hearing an odd rattling sound below the burner...then the heater stops. After a few tries, same result. Then it just stopped working, this time with a PS diagnostic code. I replaced the pressure switch. Still no go. Any ideas would help. The system has new blower, air pressure switch, igniter, and now water pressure switch and gas valve. I have no filter obstruction. Thank yo in advance.