a thought
 

We can speculate all day. How about shimming the saddle and see what happens.

I've decided I am gonna take it to a luthier. It's too expensive for me to muck around with it.

Now you're talking.

A worn nut slot (they do wear) could cause it.

A worn nut slot would not cause a string to buzz when fretted or capoed.

There's a fair chance this is a 'zip tone' issue, which is especially prevalent on the D
string- esp on nylon string guitars. Alan Carruth has some good posts on it around.

A different brand or type of D string would be one of the first things I'd try. I hesitate
to be specific, but I've seen more of these problems with Savarez basses (Corum or Cantiga),
and fewer with D' Addario ProArte- though the latter are a generally less sensitive string, as well.

True.

I missed... .. only when I use a Capo ...in the original post.

I recently corrected a D string buzz when played open and/or fretted - the slot needed to be filled and re-filed.

Thanks. I have already tried three different brands, buzz is there on all. Whatever it is, it started when I changed strings the first time. This is a 2022 guitar, so not worn, and it is in excellent condition. We are getting pummeled with snow here, soon as the roads are clear, I will take it into the luthier. It is unplayable as is.

I cannot believe it, but it turned out to be the D string zip tone issue. We tightened the windings, and that fixed it. Before we figured that out, I had the saddle shimmed, nope, and I had a new saddle built, nope. It was a simple as tightening the windings. So weird.

+1

Good to hear that it's solved.

Indeed. Thank you

a question
 

'Tightening the windings'? Wuzzat? Thanks.

This article explains it better than I could.
https://allstringsnylon.com/asn/post/d-string-buzz