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.
I forgot to mention that my all-battery radio hums too.