I've got a pump running glycol from the chiller and it makes a weird buzzing noise. One pump does it and one doesn't, so clearly something is wrong. I thought that there might be some air in the head, so I loosened the nut to vent it and the pump quieted right down - success! Unfortunately when I tightened the nut again the noise picked right back up.
Can anyone tell me what is going on and how to fix it?
Here is a pic of the head zoomed in for part numbers, the nut is on the left.