What engine? I have a 4.9 that does the same thing, the reason it drops to nothing is when it senses less then 5 psi, it drops to 0, kinda like an idiot light but in a gauge. I have ran my for 10k so far, and it still makes no noise, and ran great until I broke something else. As long as you aren't getting noise from the engine I wouldn't worry about it.
-2001 Tahoe original 5.3 4x4 only 430k
-A Metric Butt ton of three wheelers
-1985 V65 Magna
-1983 Nighthawk 650
-1969 F250 Highboy built 360