Not suitable for acoustic applications

I needed to replace the power supply for powering a cordless telephone. Unfortunately, then listen to on the phone strong noise; my first guess, simple hum, I wanted to compensate with a buffering capacitors (22,000 uF), but absolutely no effect. Presumably, the power supply is an "excellent" harmonic generator. Also test with a portable radio was similar.
I have to keep the unit, as it is suitable because of its good voltage stability for simple power supplies.