All three boilers are about 10 years old but are in a second home used on weekends and only used about 20% of the time so age is less about wear out and all three units were all cleaned and services recently. When the rumble occurs I can see the flame burning but fluctuating and smell some propane residue. This one unit has exhibited these symptoms off and on for a few years. I only recently want to find root cause and fix.
follow up question on the home made hydronic seperator, is there a reason you went with 12 inch nipple. lets say you wanted to make it fatter and shorter like 4 ix 8. Also install it on bottom like the navien or the Rinnai boilers. ( just giving myself options in my head )