Electrical interference from house wiring?

When my battery/AC and AC-only radios are connected to 120v house current, a mid-to-high continuous hum is audible on the AM band. When the battery/AC radio is running on battery power, no hum is audible.
Does anyone know what the source of the hum might be and how to eliminate it?
Thanks in advance for your helpful replies.
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Likely that the electroylics in AC part of power supply are failing. Excessive ripple voltage would cause the hum. Just one possibility. Discover if radio does same in a different venue.
Darro wrote:

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If this is from a wall wart supply it may not have any filtering at all. They all have marginal filtering. You can usually fox it with a capacitor around 30-50 ufd at 15-20v, whatever you can find hanging on the wall at the Radio Shack in this range. Be sure to get the polarity right. They make the minus look important so look closely.
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I forgot to mention that my all-battery radio hums too.
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