MR Hum

Combining hum removal, buzz removal, brickwall filtering, and spectrum analysis into one easy to use plug-in, MR Hum is the perfect tool to clean up recordings with any sort of hum or nasty buzz.
The hum removal section works by applying a set of notch filters spaced at multiples of the fundamental hum frequency, usually 60 Hz or 50 Hz.

You select the fundamental frequency, the number of harmonics you want to eliminate (up to 10), and the width of the notches (the default works fine). There are presets for the usual cases of 60 Hz hum and 50 Hz hum. You can also select any fundamental frequency continuously from 20 to 200 Hz. Sometimes it’s necessary to use a slightly different frequency in cases where the hum has been recorded on analog tape and played back on a different machine, which will cause the hum frequency to change.