Can't tell you exactly which possibility it is but if the guitar was on my bench that sound would be gone in a very few minutes.
1) Binding nuts slot
2) Too shallow a break angle in the nut slot
3) A hump in the middle of the nut slot
All of these possibilities would be eliminated simply by properly cutting the nut slots
4) Too little break angle over the saddle
5) A flat spot on the saddle permitting the vibrating wave to breach the desired break point on the saddle
6) String has worn into a soft saddle and is being pinched
All of these possibilities would be eliminated by pulling the saddle and cleaning it up with files. In the case of too little break angle over the saddle making string ramps to increase break angle or raising action both will improve break angle.
It's a common sound often heard in our shop when stuff comes in and not heard when it leaves. Can't tell you specifically which possibility or possibilities may be your noise but my bet is that addressing the things I mentioned above and you will get back the clear, shimmering high e that you wish for.
It's also no big deal and any qualified Luthier likely will track it down very quickly.
thanks so much for the reply. all good ideas. saddle had a definite flat section from string break on back to the back edge. rounding doesn't seem to have helped,maybe because the saddle's shallow enough at the low e that rounding it out did not do much to improve a pretty low break angle...
slotting could be in the offing but I think I'll chase down the nut angles before going in that direction.
I'll add to Hesh's thoughtful points - check the bottom of the saddle and bridge slot to make sure they are both flat. I've seen some real butcher jobs on saddles as of late. That being said, my bet is that the problem is in the nut slot.