Two problems related to engine clicking and tapping noises have been reported for the 2000 GMC Sierra. The most recently reported issues are listed below.
Valve tap on cold start lasts approximately one minute. GMC claims it is carbon build up in the combustion chamber and will not damage the engine. This is unsatisfactory. Any tapping sound is indicative of lack of lubrication and will prematurely wear the engine or its components particularly after warranty runs out. See service bulletin #01-06-01-028a. Or document #1271016 "engine knock on cold start".
The consumer states that there is a tapping noise in the engine. The technician at the dealer stated that when cold started, there is a light tapping noise. (found bulletin #01-06-028 for cold start engine knock) no current fix at the time and should not have an effect on the longevity of any of the engine components, and may be caused by an interaction between carbon that has formed on the piston/the piston motion and cylinder wall.