Is it making the noise all the time whether you open a tap or not? Does it make the noise when you open a tap? Was the shutoff valve opened all the way after being closed? Is the pressure relief valve drain line end attached to a drain instead of being left some inches from the floor so you can’t tell whether the relief valve is leaking by?

http://www.heatpro.us FREE heat/cool right-sizing

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.