Strange noise at higher system temps

[brant2000]

I've recently noticed a very strange noise that occurs and continues most of the time when my system is up over 170 degrees or so. It isn't noticeable at temps below that. It is an odd thumping sound, that is just slightly faster than my normal breathing rate. The really strange thing is that I can only hear it when I'm in my bedroom (of course while I'm trying to sleep) which is directly above the boiler. It is completely undetectable when you are near the boiler.

I did some piping changes during the summer, in which I moved a circulator and added a 4-way return water protection valve. My system uses two circulators and I suspect the sound may be caused by some sort of harmonic resonance occuring as a result of the two circulators.

Has anyone else ever had anything like this?

 

[ewdudley]

Could be a beat frequency of the two pumps. See if closing down a valve incrementally in line with one of the pumps changes its speed and therefore the beat frequency.

 

[brant2000]

The secondary is a three speed pump, I've tried reducing it to the second speed to see if it goes away. I think it was slightly less noticeable, but not gone. I like to run that pump on the highest speed or else I get stagnation in my storage.

It's just very odd to me that the sound only occurs at higher temps/pressures. I know that I could probably benefit from more expansion capacity, as the system pressure exceeds 30 PSI above 190 degrees; but it still doesn't make sense to me why this occurs at those pressures.