I found several posts on this topic most indicating the plastic barrier is broken in the bypass valve such as:
I have the old style (pre-04) brass, etc... My plastic piece is intact, but there was corrosion on the brass such that it wouldn't move up and down the shaft. I cleaned that up and it slides now. Put it back together, same noise. Pulled it apart again this time pulled the manifold - noticed the thermostat surround was severely corrosed and cracked on the rod above the spring. Would seem off for this to make that much of a noice, but just to see, i removed it, reassembled and fired it up and no noise.
Last thing - i have upgraded to a variable speed pump and the rpms and pressure are definately down from the 2.5HP pump i had before that used to pump up two stories of solar (no longer installed). But without the thermostat it was fine, so i suspect flow isn't my issue. On low speed the unit shuts off as i'd expect with LO indicator. Anyway doubt this is the cause , but throwing it out there.
Main question , could the thermostat truly make that much of a ruckus (i guess if it's stuck, the answer is yes?
Since i've got this apart, i probably need to ream out the burners. Should i clean the water tubes with a muriatic acid solution while at it? Any tips on either (i don't have a reaming tool yet - if something else would work)? Also, even though the unit is on a pad, the bottom steel case around the burners is starting to flake/rust. Assuming that's normal where rain water would pool up?
thx in advance