Oil Pressure Klunk?

[DAldous]

Help! About 25% of the time, when I start my engine (the first time each day), I get a fairly loud "klunking" sound when the oil pressure "kicks in". (It sounds like the computer shuts the engine down momentarily!?) The truck is a 2000 cummins with 44,000 miles on it.

 

[TBrown]

My truck will do the same thing once in a while, but only when the AC or defrost is turned on. It is the clutch engaging on the compressor as far as I can tell since it never does it if I have it on vent or heat. Doesn't sound good, but nothing seems to be wrong.