All good advice so far, to which I'd add:

De-humming guitar (or anything else) is often best achieved by combining several tweaks in moderation rather than a single, powerful one. Broad-band hum is harder to get rid of than a mains-only one, and induced noise caused by a PC or lighting dimmer can be particularly difficult to counter in the mix. It's generally better and easier to tackle the noise at the source than remove it later. Often simply moving the guitar somewhere else in the room, or even the guitarist facing a different way can lower the noise floor. As can a change of guitar lead (and the shorter the better). And make sure the guitar's string earth is working properly.

However, once the noise is tracked it has to be minimised.

So:

High-pass with a steep slope at around 85Hz.
Watch for noise at multiples of the mains frequency, particularly the first couple of octave harmonics around 100/120 and 200/240Hz depending on your local mains AC frequency. Reduce the noise with narrow band eq as far as possible without the eq wrecking the overall sound.

Careful, gentle use of a post eq gate/expander can work wonders.

Consider lowering the volume of the track until the hum isn't objectionable then build the rest of the mix around that. With any luck once the entire mix is then brought up to final master levels any residual hum on the guitar track won't be noticeable.

If all else fails, if the problem is mostly noise in the gaps between guitar notes, especially if th guitar was recorded with no or minimal reverb it is possible to go through the audio, chop it up so the noisy gaps between notes are isolated then reduce the noise-only clips to silence with a bit of a fade-in and out, if necessary gently shortening reverb tails as needed. It's a slow, painstaking job though.

Bill said "Cedar".
 
:-)
 
Yeah man!

Only apply any form of noise reduction in the areas where you can hear the problem. Leave the rest alone. Noise reduction can alter the sound to some degree as well. If you cannot hear it then leave it alone! You will find there are only a few areas where the noise or problem is audible.

Thanks for the tips guys. The noise is across multiple frequencies, but I used the Waves LinEQ to perform a narrow Q deep cut at 3618 and that took out the harshest noise. It's only noticeable in clean parts of a cover of Jolene. I think I'll chop up the clips and apply the EQ only to those clean parts, and not to the entire guitar track.
 
On a related note, is there a good reference chart for hi pass/low pass filters for various instruments? I'm always afraid I'm cutting out too much (such as when hi-passing a guitar) but in reality I'm probably not cutting out enough...

3618hz is not what i would call 'hum'.

The frequencies to hi/low pass at depend very much on the music in question.

There are a few obvious principles, such as a guitar in standard tuning doesn't go below roughly 85Hz or a bass below 45Hz so anything below that is noise so it can go, guitar speakers tend not to do much over c. 8KHz etc.

You might find this chart useful - http://solomonsmusic.net/musrange.gif

However, one reason for lo/hi/band passing is to carve space in the mix. For example, if a guitar is busy around 80-240 Hz and the bass is playing in the same region they can trample over each other very badly. Hi-passing the guitar to reduce its low frequency content might help clear things up. Same with a bass and kick drum if their frequency range overlaps.

A frequency analyser plugin can be a very useful tool for finding overlapping instruments/voices and creating space in the mix for them.

Bats - perhaps hiss or sizzle would have been more accurate than hum. :) At any rate, thanks for the advice - the LEQ helped.
 
Thanks for the chart tlw, that could prove to be very helpful...

ELECTRICS
 
the bane of my existence.