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I thought I would throw this one out there as is is a new one for me.
This is a domestic recirc pipe that has run at excessive velocity for 20ish years. The building has had many leaks where you would expect on 90's and tee's.
It is also pin holing on straight sections of pipes. Check out the inside of the pipe.
"If you can't explain it simply, you don't understand it well enough"