Long time lurker, first time poster.
I have a MasterTemp 250 NGas heater that's been giving me some headaches lately. Last year prior to closing it was 'bumping' and rattling. This spring I've replaced the thermal regulator and the manifold bypass valve (thanks to posts on this site). I believe the 'bumping' is gone, however, there still appears to be some rattling. I had the heater on today, but at some point it quit heating but it kept running. I noticed the 'heating' light was flashing. I also noticed the 'AFS' light briefly came on once but did not stay on, nor did I see it a second time.
To me the rattling seems to be coming from the 'combination gas control valve'. There appears to be a switch on the side closest the opening with 2 wires going to it, maybe a breaker of some type, this seems to float back and forth. The part is attached along with red arrow pointing to switch I am speaking about.