Eliminate humming noise?

I think it's safe to say we need to know more about your setup before we can answer the question. This info should be sufficient;
What kind of Mic do you use?
What is the input device you are using? (Are you using a USB mic, are you using a recording interface, if so what brand? Are you plugging directly into an analogue mixer and then into the 1/8" input?) Also, as a point of interest, what kind of cables are you using? Is everything solidly plugged in? Sometimes when cables are not all the way in, that can produce feedback. Also, when you record, you should monitor yourself while you listen in headphones to know whether you are getting a hum or feedback. (To do this, open your project and look to the track info menu on the right, if you have not selected the details tab [for effets etc] you should see an option that says "Monitor" at the bottom, and I would recommend turning it to "On with Feedback Protection") If you already do that, that is a bit more interesting, if you didn't hear it while monitoring.

Hm, it sounds like it might be a problem that is originating from your headset. The only thing I could think of would be that your headset may be too close to speakers, if you use speakers while recording and it's getting feedback if your monitoring on. That may be slightly unlikely, but I can't think of much else. i would also make sure that the USB chord is fastened securely. You may want to consider using the built in mic of your laptop, if it turns out to be your headset.

make sure all your equipment is plugged into the same grounded outlet. also if you're near any heavy appliances, or even florescent lighting, these can cause hum.

I have that same brand USB mic/headphone set. Its really great except for that one small humming noise you hear when recording. I was able to identify that it was NOT the external hard drive, or my iPhone I had connected at the same time but rather the laptop itself. I use a powerbook g4 and when I am recording WHILE touching the laptop it does NOT make the humming sound. But as soon as I stop touching the laptop it starts up with the noise. Im constantly monitoring my track so Im always touching the laptop even while recording so its no big deal for me.

I'm assuming it has to do with the EMF with our bodies + the electronics. Either that and/or the shielding on the USB mic/headphone set isn't that great but anyway, that's what did the trick for me. Hope that helps.

@sketchstudios