Thanks for your response. My "audio" skills are minimal. Re the DeHummer effect - how does one determine the Number of Harmonics and Harmonic Slope to use? I suspect the way to do it is check "Output Hum Only," listen to this clip and change the variables until you think all the hum is identified. But, this is a guess on my part, is there a more rigorous way to determine how to correct the hum problem?
If you open the Frequency Analysis window and make sure that the Scale (top left box) is set to Logarithmic you should be able to see spikes at the hum frequency and it's harmonics. From this (with a bit of practice) you can see how many harmonics there are and how much attenuation may be needed. The hardest thing is to accurately find the Base filter frequency and dial it in in the DeHummer page. Some experimentation with the number and reduction of harmonics will be required.
Hum with lots of harmonics sounds more "buzzy" (like a bee) than a pure single frquency hum. And yes listening to Output Hum Only will help you tune into it better.
As ryclark says, the frequency analysis window can really help you find the base frequency for the hum--if you can find a piece of your recording with no actual sound content (just the hum and other background noises) this becomes much easier to pick out than trying to see it in amongst voice or music or whatever.
I would generally start looking around 50Hz or 60Hz depending on the mains frequency where you live. There are exceptions, but hum is very often mains electricity related.
As for the harmonics, they can be hard to see in the Frequency anlysis window--if they're easy to see they must be pretty severe. I usually rely on my ears to determine the best compromise in terms of removing hum vs. removing frequencies with real progamme content--the harmonics are getting up into frequencies shared with the human voice and many instruments.
Of course, the best advice (as always) is to try and remove the hum at source next time. Using balanced instead of unbalanced connections, running all gear off the same ground/earth point, checking the soldering in your connectors, routing audio cables away from mains cables, isolating transformers, etc. can generally fix any problems before you have to resort to the de-hummer.