I am getting a high pitched whistle from my pipes. Whenever the circulator pumps turns on I hear it. I have written in this forum about this before and have done many things to correct it. I replaced the baseboard. The noise went away only to come back two days later. I swapped circulator pumps. Noise came back a week later. I was trying to pin point the noise and seems to be coming either right before or after flow check valve. Whenever I shake pipe or hold pressure on pipe the pitch of the noise changes. I have a closed loop system. Any help will be much appreciated I am going bonkers trying to fix this.

coming either right before or after flow check valve.

Eliminate the valve and test the system or just replace the valve since you have done almost everything else. It could be valve chatter. If installing new valve pay attention to flow arrow so it is insrtalled in the correct direction.

There may be some loose obstruction in the piping, and the repeated emptying of water for repairs causes it to temporarily migrate away from the point of restriction, only to migrate back when flow is restored. If so, this could be a real problem to locate and remove. Got a stethoscope?