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It is almost certainly a switch mode power supply. You can try plugging that supply in through a power line noise suppressor (keep the cord between the power supply and suppressor as short as you can so it don't act as an antenna), add extra filter capacitance to the output, and extra metal shielding around it, but the benefits of that are limited. Switch mode supplies are basically oscillators controlling power circuitry, and are MASSIVE noise generators. If you can't suppress the noise from it, then look into going back to the wall warts, or get a 120V-12V line step down transformer, a diode bridge rectifier, two good size 25V caps, a 12V regulator chip (if you are using it as a 12VDC supply), and build your own noise-less linear power supply (it is fairly easy).
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