As mentioned above I bet it's the sympathetic resonance of the strings between the nut and the tuning pegs. I get that as well. Put some cotton up there to see if it goes away. I know it doesn't answer the question ("how to get rid of it") but at least next time you'll know what to do to not get it in the first place.

One tool you might find useful is Voxengo Span. It can be used to identify the frequency of any sound you hear. Once you know the frequencies you're having problems with it makes it easier to deal with them.