PAPicker is right, usually misaligned frets are the culprit.
If you look down the neck from the nut towards the bridge and hold down the low E string at the fret closest to the body, you can see the entire neck profile with a straight line (the E string) to compare it to.
If it's pretty bad, you can sometimes see one fret that may be a little higher than the rest. Or...
Occasionally, and this is pretty rare, if your front (nearest the neck) and (maybe)middle pickups are very high, the mass of the thicker strings is great enough to be attracted to the magnets in the pickups, and they get this weird sound - I wouldn't call it buzzing, but you might. It is actually the strings rattling against the polepieces - it doesn't happen when you're playing further down the neck, because the strings are not as close to the pickups when you push them down.
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