Sounds to me like you have 2 issues...
The hum could be ground loop. Check your power cables and interconnects. Try to isolate as much as possible, and if they have to cross, do so at right angles. I have an API Power Wedge and Synergistic/Harmonic power cords, but even then I occasionally have had hum issues. Simply moving them around reduced the audible hum.
If you started hearing the cracklies upon adding the MSD4, I'd say it probably has an issue of some sort that needs fixing inside.
The hum could be ground loop. Check your power cables and interconnects. Try to isolate as much as possible, and if they have to cross, do so at right angles. I have an API Power Wedge and Synergistic/Harmonic power cords, but even then I occasionally have had hum issues. Simply moving them around reduced the audible hum.
If you started hearing the cracklies upon adding the MSD4, I'd say it probably has an issue of some sort that needs fixing inside.