Just a guess, but it could be something to do with the driver. Hissing=high frequencies present in the current, in addition to the usual chopping frequency (above audible range on any worthwhile driver) and stepping frequency. It maybe caused by the driver directly, or the motor causing the driver to have an effect on the motor. It's something that can be caused by poor design of the stepper driver, but clearly that's not the case here if it's run without the sound previously. Perhaps try the motor on a different driver? You could also verify that the holding torque of the motor is still close to rated, as that's quite easy to test. It could be some other machine/device interfering with the signals somewhere - I think I've had that happen before (although again seems unlikely as it's generally the motors that interfere with everything else, not the other way round).