My 99 Altima (93,000 km) has had a tapping sound for the first few seconds
when I start it cold since I got it.  In the warmer weather it seems more
apparent immediately after startup.
I've had the dealer listen for it a few times now, but they've never been
able to hear anything unusual.  That's understandable partly because it
usually doesn't happen unless the car's been sitting for some time, although
it can be a little hit-and-miss as to when it does it and when it doesn't.

Within 3 seconds or so of starting, the sound is usually almost totally
gone, but I can often hear a slight tap for a while after startup.  In the
winter, I could hear a tapping as I drove when the revs approached 2,000 rpm
for the first few minutes of driving, and during cooler weather (say less
than 10 degrees C) I can often hear a bit of a tapping/rattling sound around
2500 rpm under moderate throttle.  This may or may not be related
(obviously, I'm just guessing here).

Also, if I put my ear down close to the valve cover, on the flywheel side of
the engine, toward the front (exhaust) valves, I can hear a tapping in that
location that is louder than anywhere else under the valve cover, *and* it
sounds pretty similar to the startup tapping.  I haven't yet been able to
get someone to start the car while I listen there to see if that's where the
sound is coming from on startup.

What's really got me thinking it's a valve adjustment problem is that a few
times now I've pulled the fuel pump fuse and cranked the starter over for
5-10 seconds to get the oil circulating and see if I still got the tapping.
So far I have not heard it after doing this.

What I'm wondering is:

- Is there anything other than a valve out of adjustment that might act and
sound like this?
- I've heard many times that solid lifters never need adjusting, but the
manual suggests adjusting them if they are noisy, is this uncommon?
- Am I likely to be doing any damage if there is that much play in one of
the valves?
- Can I check the valve clearances in the driveway with a feeler gauge if I
pull the valve cover?
- Should I go to the dealer and ask them to check/adjust the valves and is
that a time consuming (ie expensive) job?

I'd really like to clear up the startup noise just to see if the other
noises I mentioned go away -- that would ease my paranoia a fair bit :)

Thanks for any suggestions.