FAQ02079 of Solid-state Relays FAQ

Leakage current flows when the Solid-state Relay input is OFF. If a small Relay is driven, this Leakage current may cause the Relay coil to become slightly energized, which results in a humming sound being produced.

Insert bleeder resistance in parallel with the Relay coil to eliminate any humming. The same effect as bleeder resistance can also be obtained by connecting multiple Relay coils in parallel or connecting the Relay coil in parallel with another load.