I have a 1999 Altima (110,000 km) that has always had a tapping sound for 1
or 2 seconds after I start it, usually only on warm mornings when the car
has set overnight.  I've never been able to figure out what it is, but the
dealer suggested it might be a hairline crack in the exhaust manifold;
apparently that was a common problem with this era Altima.  Whatever it is,
it hasn't gotten any worse in over 2 years of having had the car, so I've
just learned to ignore it.

I did have the valve cover off this summer to replace the gasket, and
everything under there looked brand new; no scoring on the cams, no sludge,
and the timing chain was tight and looked to be in great shape, so that
helps me sleep at night ;)

Hope that helps!

I just bought a 1999 1.6L Sentra.  Runs great, engine smooth and quiet.
I do occasionaly hear a slight metallic sound for a second or two
during start-up on a cold engine.  Can anyone describe the
symptoms/sounds associated with a timing chain going bad?  Thanks.