Is it the window panes rattling within the sashes, or (more likely) the window sashes themselves rattling against the frames? The former might be helped by reglazing (and you will actually see gap between the glass and the sash if you rattle it yourself), but I'd guess it's the latter, since that's super common in older houses. You can add insulation to the windows and win on two fronts: less rattling and fewer drafts. We used spring bronze weatherstripping on ours but there are lots of options, some simpler to install than others depending on your windows. The weatherstripping isn't visible from the outside but helps the window sit more securely in the frame. Should decrease rattling substantially.

It's not a structural problem. It's an old house and things settle and sometimes loosen up. Is it the glass in the frames rattling, or simply the windows themselves? If it's the glass, re-puttying would probably fix it. If it's the frame, it could be:

1) Wear from years of use that has loosened things up. Only fix would be to place a small wedge between the frames to hold things firmly or replace the window.

2) The hidden sash weights rattling (might not be fixable).

3) The latches have loosened and up need to be screwed down firmly or replaced.

tuck backer rod between window sections.

If it's the frames, felt weatherstripping may do the trick too.

jlc-
Ok...you've been given lots of suggestions. How about telling us if any of them will help your situation?