That is fairly normal. As long as the engine has oil pressure above 4-7 PSIG, the pump will operate. Since the oil pressure must bleed off through bearings after the engine is shut off, it can take a few seconds, and even longer if the oil is cooler or more viscous. Mine does the same thing. As a matter of fact, all my vehicles do that, sometimes for 7-10 seconds in cooler weather.
If it really bothers you, you can remove the oil pan, install some worn out bearing shells (I can get you a set), and the oil pressure will bleed off faster after the engine shuts off so you don't have to listen to that nasty pump sound any more.
If the pump stops and starts again, that's a different issue. That could indicate a sticking/failing oil pressure switch, or an accumulation of sludge in the oil passage to the switch. Your's might be located near the distributor, right near and under the one for the oil pressure gauge. Since you have a later engine, it also might be mounted near the oil filter/cooler adapter. The switch you are looking for should have a large gauge orange and a large tan/white wire connected to it.
