Lighter strings can cause a buzz for 2 reasons:
1. Because they require less tension to tune to pitch (btw, concert 440 is 'normal' tuning, E, A, D, G, B, E. Some people use alternate tunings and for the sake of guitar repair or tech tips, it's good that we all start at the same place, what the instrument was designed for. That's not to say that some guitars don't sound or play good, or even better, with alternate tunings but 440 is about the best reference point from which all adjustments are made.) it's possible that your headstock/trussrod is pulling back more and giving you less relief than you had before. You've checked your relief and say it's good but maybe it was a tad higher before?
2. Lighter strings, because they tune to pitch with less tension, vibrate in a larger arc requiring more clearance space from the frets to keep from buzzing.
There's 2 methods of approach I'd take here. One is to get a good straight edge, like a metal or even a plastic ruler that's only about 6 inches long or so. Place the ruler on the frets inbetween the 5th and 6th strings with the center of the ruler on the 12th fret. Now see if the ruler will rock back and forth using the 12th fret as a pivot, like a see-saw on a playground. Obviously, the movement won't be as much but you're checking to see if your 12th fret is higher than the other frets. If it is, take it in to a shop to have them level and dress it.
If the 12th fret isn't high and your action can tolerate it, I'd attempt to adjust the saddles up a bit to kill the buzzing. Well, I say that assuming it's an electric. Is this an electric?
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