It doesn,t rattle all the time sometime it will rattle and some not
5 minutes after i start the car i hear slight rattling in the engine sometime on 2000 Lincoln Town Car
Without the benifit of hearing the noise it would be guessing. Could be a timing chain tensioner losing oil pressure. Common problem.
After it kicks into low idle, the vibration of your car changes and you are probably hearing the heat shield on the catalytic converter shaking. Probably broke a weld with age but, this is not a serious prob. just annoying. You can get it rewelded on at a shop.