rattling

The timing chain and tensioner and hydraulic plunger may have,just enough wear that when the oil temperature goes up to normal,and the oil tends to thin out or loose some pressure the rattleing chain or excess slack will be audible.It may get so you here it with cold engine as well.

100% Chance it's the chain guide loose, broken or the chain hitting against the front engine cover. Get into it quickly and replace the chain sprockets, guides and tensioner. I've just spent time on my D21 and it matches your problem. If you don't get to it quickly the chain will chew a hole in the waterpump jacket and you'll get water in the oil and think you've blown a head gasket. Take off the valve cover and look for shinny spots on the from cover plate.