Sullair 375h compressor making noise when holding pressure

Hello I'm getting excessive nose coming from three compressor after it builds pressure but only if the pressure is retained, if i open the service valve the noise goes away as long as it stays open the noise stops, as soon as it is shot off it returns to making a loud humming noise. Checked fluid good, changed three filter, (compressor oil filter) removed and cleaned the thermal valve don't know what else to check could you please help?