harmonic distortion

[hvac01453]

I have been servicing an old Lennox heaper 80's vintage... and have a harmonic distortion that hums in the space. It only happens when the refrigeration circuit is operating. I even disconnected the condenser fan motor in the event vibration was comming from there. I installed a muffler in the event it was pulsations, to no avail. The only other thing I was thinking it cold be, is the metering device...I doubt the dryer would do that...Oh...the unit was low on charge and when up to full charge, the hum was even louder...I checked the rubber mounts on the compressor and they seem fine. I don't feel any vibration in the tubing with my hands either... any ideas? mad:

[750 Valve]

Dont know if this is of any help, had an epr (ORIT10) in a meat case that had only approx 2 psi press drop across valve, it used to intermittently resonate or "sing". Changed valve, new one does it too... epr is now wound out and meat a little bit colder