Last year, I moved into a home with a pre-existing concrete pool and spa (both connected to the main system, separated with Jandy valves). Among its equipment is a Sta Rite Max E Therm 400 heater, which is the problem child today.
The heater runs correctly when the system is set to filter only the spa. When the system is only running the spa, the pressure is much higher - approximately 30-35 PSI - because it only has one output for water to flow into the system, as opposed to 4 for the main pool. It typically runs about 20-22 PSI for the main pool.
When I have it set for the pool, it will run correctly for a seemingly random period of time.. sometimes an hour, sometimes 3 hours, and then it will make a fairly terrible noise as shown in the video below, do that noise for around 20-30 seconds, and fail with a "service heater" light. I've replaced several things on the heater because they were easily accessible and old - thermistor, water pressure switch, and thermostat/spring (whatever the term is - looks like a small car thermostat).
In fairness, it's possible that the spa setting would eventually fail as well. I've been in the spa for a couple of hours at times and never encountered it, but I've never tried running it for a full day or anything.
When it fails, it does appear to do a proper shutdown (the "vent" sound continues for a little bit, presumably to cool the heater down), and not some quick "panic" shutdown.
Here's the link to Youtube I managed to capture (sorry for the poor videography):
Here's a video of the problem actually happening.
Please don't pay attention to the pressure display on the pump during the video, as I was quickly trying to see if I could make the heater *not* fail by switching the system to spa very quickly, raising the PSI. While that noise was going on, the heater seemed to be physically vibrating/banging - I could feel it in the gas line and the heater shell, in addition to (obviously) hearing it.
Edit: Once "Service Heater" comes on (approx 45 seconds into video), the noise stops.
I'd love any thoughts people have on how to fix this. I'm still quite a novice with pools having never owned one before.