I notice the same thing on my GS when starting or shutting off the engine. It's very brief. It is symptomatic of older "lifters" (automatic hydraulic lash adjusters), like over 75K miles. I've heard that replacing the lifters fixes this and restores some power, but I've also heard that it costs at least $1500, and my honda/acura mechanic sez the difference is minimal, especially considering price.
Look on the bright side - they save you from having to do periodic valve adjustments yourself!
as they wear, they "leak down" a bit after sitting or at a low speed idle. When oil pressure is increased they are fine...as you said, it is not worth it to replace them, unless one completely collapses and makes a racket....then just do the failed one.