It sounds like armature noise (the moving part of the relay that is attracted by the coil).
Normally there are two reasons, too low a voltage or bad seating of the armature, but this is usually prevalent in AC relays.
Do you have 24vdc on them?
As even raw DC (unsmoothed) does not cause this, unless maybe 1/2 wave rectified DC.
Just for kicks you could put a 100 to 500µfd across the coil to see if it stops , observing polarity, but make sure it is DC!.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.