Here's the method I used to quiet the lifter tick on my 4g64. The process is the same for the 6g72. This may not work on older generations because the lash adjusters were actually changed in 1999. Follow the directions exactly. You may want to print this out so you know the levels and durations for each of the revs. It seems strange, but it really does work.

When you start you car, there is a noticeable "ticking" sound from under the hood. Upon opening the hood your ear pinpoints the noise as coming from under the valve covers. If you were to open the oil cap while the car is running (not recommended since oil will splash out) you will see the lifters, rockers, springs, etc. and will notice that the ticking gets louder since this is the source of the problem.

First, check your oil level. As dumb as this sounds, the ticking can be compunded by low oil level. If the dipstick is OK, then follow this procedure:

1) Start the car.

2) Rev to 3,000 rpm for about 20-30 seconds.

3) Let the car idle.

4) Listen for ticking sound.

5) If still ticking then rev to 6,000 rpm and hold for 15 seconds.

6) Lower the revs to 5,500 rpm and hold for 15 seconds.

7) Lower the revs to 5,000 rpm and hold for 15 seconds.

8 ) Lower the revs to 4500, 4000, 3500, 3000, 2500, 2000, 1500, and 1000 rpm and hold each level for 15 seconds.

9) Lift off and return to idle (750 rpm).

Note: This makes sure that the engine goes through a good range of revs and returns slowly..., so as to not introduce any more air bubbles that you are trying to remove.

Ticking still present?

Time to change out the weight of oil. Buy some really lightweight oil. Put the lightweight stuff in for this exercise instead of your normal stuff. The idea is to get some really lightweight oil to get into all the little crevices and get out all the contaminants. Then after a few hundred miles drain out the lightweight stuff and put in a good synthetic oil (Valvoline, Mobil 1, RedLine, etc.) to the appropriate level and see if the tick has gone away.