Engine noise

2002 Subaru Forester, 2.5L engine rattles/knocks at start-up when cold. Noise goes away after engine heats all the way up to operating temperature. Then it runs great without any noise. Sounds like something on the outside of the engine in front (timing belt, water pump, belt idlers, alternator, Power steering pump or such). Doesn't sound like anything internal (pistons, conn. Rod, wrist pin, main bearings, etc.). Just drove it on a 500 mile trip without noise (except at start-up) and it ran perfectly. I tried an oil additive that is supposed to free up valve lifters but that didn't help. Have you heard of this before? Any idea what it could be?