Brake Levers? My DiaCompe 188s rattle pretty good, more so now after WetFlower.

presta valve rattling against the rim? there are tons of other possibilities. you could tape down the things that you think might be rattling to figure it out.

Shifter cables?

Headset tight?

I second the headset check

Have you got bar end shifters? My rear shifter rattles any time I'm not riding on the aero bars. Drives me crazy, but I can't find a fix.