There are several things that can be causing the problem, go through the list below to see if you can narrow down the cause of the issue. It may be that that issue is always there, and that you can only hear it when using headphones, or it maybe that the headphones themselves are to blame:
- Try a different pair of headphones to see if it's the heaphones themselves that are at fault. If you still occasionally hear the buzzing, then it'll be something else in your setup. If it's your headphones themselves, then there'll likely be a loose connection within the jack, or some other part of your headphones. If this is the case then you're probably best sending them back for replacement / buying a new pair, etc.
If it's not the headphones, then:
Try swapping out your M-Audio interface for another, even just your PCs onboard soundcard, to see if the problem persists.
Try changing the USB cable that you're using to connect the interface to your PC.
Look for potential causes of electrical interference that could be introducing the noise. eg. Sometimes having power supplies near unbalanced audio cables will introduce such interference.
Check if any other audio inputs to your computer could be introducing the issue. For example, I've got an XBox connected to my LineIn on my computer that introduces low-level noise when I choose to play it back through my standard computer speaker output, even when the XBox itself is off. I've also had a screen that had a dodgy power connection, that would cause a buzzing noise to come through the speakers when it was turned on.
Once you've found out exactly what's causing the issue, it'll be easier to come up with a solution.