Very Odd Problem
I was playing my acoustic earlier and everything was going fine. I decided to throw the strap on and walk around the house and play. Put it on, fretted some chord and noticed it was buzzing terribly on the first fret. Sat down, fretted the same chord, same way, same pressure-No buzz. Stood up again-Buzz.
What the....? :?
Where does your strap attach to the top of the guitar? Body or neck?
I'm only guessing here, but could the neck be loose? Maybe the strain of the strap on the neck (if strap mounts there) got the neck slightly out of wack?
"Rock And Roll Ain't Noise Pollution"
Unless you are playing with your guitar in ultra-dork mode, more than likely you've changed wrist angle. I mean if you have your strap set so your guitar is up higher than grandpa's pants your wrist angle may be the same, otherwise take a look at your wrist.
Try fretting the chord standing up and keep it there as you sit. See what happens.
My guess: the sudden buzz is from somthing hitting the string. I'm with Nick on looking at the wrist angle... about a year ago I had a buzz I couldn't figure out coming from my Martin every once in a while. Simply couldn't figure it out. The next day it wasn't there, or the next... it was probably a week and a half before I heard it again, and I STILL couldn't figure it out - it didn't happen on every note.
Another couple weeks later I discovered the cause. It was a new shirt.
The cuff buttons were in just the right position to sometimes bump against a string right at the bridge.
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The strap attaches at the neck, so that may be a possibility.
I "wear" the guitar in the same position as when Im sitting, slightly lower. I did the fret and stand up thing and it still did it. Even just singularly fretting the high E it does it. So I doubt it's my wrist.
I was wearing a t shirt and no bracelets or rings, so nothing was hitting.
With the strap attached to the neck you can expect a little more back pull on the neck. So add to that your pull on the neck when fretting I think we may have found the problem. Try not to pull on the neck or press hard with your other arm on the body of the guitar and see what happens.
Typically guitars with neck straps need a little more neck relief to compensate for the extra pressure.