Cast iron boiler kettling....now what?

I am just a homeowner and have gotten great advice here over the years. Most recently I posted about my 25-year-old Weil McClain cast iron boiler (hot water radiator system) making a kettling sound. I tried a couple of recommended products, one from Axiom and one from Sentinel, but neither seemed to help. So, my question is could I be missing something?

I've got a primary/secondary piping arrangement and have had the circuits isolated with the primary pump hot-wired to run 24/7 with the additives. Every day I fire up the boiler and let the water heat up to about 150 deg, then just let the pump run the rest of the day and letting the temp return to normal. Ran it that way with the Axiom product for two weeks, power flushed it, and then tried the Sentinel product for two weeks. The sound hasn't improved and may even be a little louder now. It is the worst when the boiler fires and the temp is rising from about 120 deg to 140 deg. Then it seems to quiet down a little, but it doesn't go away.

I'm sure there is no air in the loop and there are no leaks. Is there anything else that can be causing the problem? For example, could it be corrosion/rust/build up on the outside of heat exchanger? Is it possible that the gas flow to the burners needs to be adjusted or the regulator replaced?

Perhaps it is time to bite the bullet and replace the boiler. But before I go to that expense I thought I'd reach out one more time to see if anyone has other thoughts.

As always, your help is much appreciated!