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My Jumbo always had a med to high action so I decided to lower it. In the process I created a string buzz. It doesn't do it as I move up the neck, only on the open B string.

Now before you say that it's hitting the first fret let me clarify that it is not. The string height is above .012" off the first fret and having the feeler gauge in there when I pluck the string does not affect the buzz. I've check the ball end of the string and it's seated just fine. When the action was only slightly higher this sound was not to be heard. The slot in the nut is the only adjustment I've made.

And "buzz" may not be the correct term, it's more of a weird,tinny,rattly sort of thing.

Any ideas on what I just did and how to remedy it?

Usually this means there is a "hump" in the slot rounded instead of perfectly flat slanted at the same angle as the headstock. The slot may also be a tad wide as well. CA and baking soda or bone dust can be used to fill in the slot for a re-cut.

One thing we were taught at Martin is the the nut slot isn't flat but have a slight radius on the bottom. Wire doesn't bend on a given point so think how the string will roll . Just a slight curve is all it takes.
I was at martin for the 1st repair summit . there were 60 of us there and we were treated like kings for 2 days at the repair center and the plant. We had a chance to exchange information and business information .
All I can say is how nice and high class CF Martin is .

Not a real buzz but a sitar type sound....? The same thing can happen at the saddle just for info sake.
Tom

Ken, That was it. I must have been moving the file up and down when I was deepening the slot and caused a hump. Anyway, it's OK now. Thanks for everyone's help.
Tony, I've been to that site but never saw that link. Thanks for the heads up.

Tom, Sitar is the sound! Can you explain how the saddle may cause such a sound? This whole set up part is my least favorite and the more I know, the better I can get. (Hopefully)

Excellent!

Those strange noises can really get to you.

Another thing that can be done when cutting the slots (not shown in my tutor) is the actually angle the slot toward the appropriate tuner post rather then perpendicular to the nut edge. This angle allows the strings to slide in the slot more easily by making the turns a little smoother.

If the saddle tends to have a flatish top with the string coming off the back part of the saddle and a small space between the string and top of saddle on the front edge, it can tend to make the sitar noise. Top of saddle should have a roundish shape which is dependent on where you want the take off point to be based on intonation.
Tom