I have copied an answer posted on Stack Exchange to another website (respecting the SE license, viz. cc by-sa 3.0 with attribution required). Is it better to leave a comment notifying the Stack Exchange user that I copied his/her answer, with a link to the other website? Or would such a comment be considered as non-relevant?
I don't think this is necessary provided you comply with the CC BY-SA 3.0 license terms.
Adding a comment that doesn't request clarification, provide constructive criticism or add information relevant to the answer is noise, in my opinion. I don't consider notifying an author that their answer is posted elsewhere relevant to the answer. Why does it matter where it's been copied?
In my opinion, yes you can.
- The author of the answer might find personal interest in seeing it posted elsewhere.
- Other people might also find it useful, since there might be useful comments and/or related posts in that other place.
I don't consider this noise, but keep in mind that others do, so the comment might vanish one day.