Humming is typically indicative of a mechanical bind the ELR solenoid assembly cannot overcome. This could be the latches rubbing the strike, (if there are vertical rods) the rods not centered or slightly askew or the top rod is out of adjustment.
The pin on the yoke assembly is either misaligned, rubbing or burred. (See Image in Additional Information Section Below)
Humming can also be from too much resistance in the wiring. to confirm this,with the power to the ELR150 power supply turned off check on pins 1&2 (white wires) on terminal block 2, check the ohms, it should be between 4.5 - 6 ohms.
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