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@bjohnhy - okay, time to play 'fantasy plumbing'!

• I have actually come to appreciate some of the simplicity of non-condensing cast iron boilers. If I were going to stick with CI, the Weil-McLain CGa-25 is pretty much perfect. It's rated for 38K BTU/hr input, approximately 30K BTU/hr output. If I understand hydronics correctly, my installed radiation can actually dissipate 30K BTU/hr with 120F average water temperature, so I would probably need to re-plumb for primary/secondary piping to avoid having it in the condensing range whenever more than 1 zone was calling for heat, but this setup would be simple, reliable, and heat the house just fine.
• If I wanted to max out efficiency but stick with gas, I would probably go for the smallest modcon boiler I could find that still had a 10:1 turndown ratio. I would also love to switch my 1st floor from baseboard to in-floor heating. That floor has the highest heat loss by quite a bit, and already has a lower radiation:heat loss ratio than the other 2 floors by about 50% (and the baseboards are frequently blocked by curtains or furniture). I don't have a particular model in mind, but having lots of settings to tweak would be fun.
• These things last for decades, so my preference would actually be to switch to an air-to-water heat pump and drop the gas connection to my house entirely. Just like with the modcon, my oversized radiation can really shine here. I'd also go with the in-floor heat for the 1st floor again (or just add more baseboard if the floor wasn't practical for some reason). I think a nominally 5-ton unit with EVI and an inverter-driven compressor would be super efficient and heat the home just fine, and also come with lots of fun settings to tweak. I'm not sure I can find anyone around here to install/service one, but maybe some day.

Also the higher the temperature, the more prone to cavitation.

3 guidelines if you suspect cavitation.

Keep system pressure high as possible

Lowest fluid temperature need to heat the home

Pump away from the PONPC.

Since you are pumping at the expansion tank, no easy option to change that??, the pressure at the circ's inlet is dropping and you get closer to the boiling point based on both pressure and temperature.

The graph attached a few posts up shows this.

Lets assume you run 170 fluid temperature, at that temperature you need about 8 psi at the circulator to avoid boiling or cavitating.

A gauge at the inlet of the circ would answer the question. If you have a static, no fluid moving pressure of 12 psi. The pressure drop around the circuit being circulated is WAG 6 psi. So the circulator inlet pressure would be 12-6=6 psi

Use that number on the left side of the graph below

Run across the graph to the right to see the fluid temperature intersection.

So the slightly higher head circ that you added, the fact you are pumping at the PONPC and your static pressure and highest operating temperature all factor to answer the question what could be causing the noise at higher temperatures.

You need to fill in some blanks, at what temperature do you start to hear the noise?

Also the pressure at the circulator inlet.That could be measured, or calculated with a few steps of number crunching.

Simpler yet, increase fill pressure and or reduce operating temperature. Does the noise disappear, then you have a cross check to what may be going on.

Yep, the question is unanswered.

Is this a pump swap, or just a gasket/ leak repair?

Flange replacement may not be an option.

Grundfos and Wilos run about 1/8" wider flange to flange than Taco, but the piping usually allows enough movement to accommodate that 1/8" difference.

Replacement bolts are 7/16 X 2" length. Chrome or zinc plated are best. Available at most any hardware of home "box" store.

I'm pretty sure you would need flat gaskets with those flanges.

I would not attempt any repair or removal without new gaskets on hand.

the first car looks like a 2024 Yenko Camaro, HP is crazy on this things 😳
The 1970 Chevelle Super Sport is very nice.
This is the pinnacle of old school man-cave.

Your gas bills do not seem that bad. Where are you located?

What I did with my boiler was to mount a strap on aquastat to bypass the circ relay. When the stat was satisfied the burner shut down and I kept the circ on until the boiler cooled to 125 or so. It would overshoot the stat about 1 degree.