oil pressure

I am the new owner of a 1993 190e 2.6. When I start the engine from cold, the oil pressure takes a couple of seconds before it starts to rise. During this period of time, I can hear tapping noises. Once the pressure begins to rise, the engine is very quiet. The pump seems to be working well as the presure is good and remains good during all driving situations. I have just changed the oil with 15W-40 oil & a new MB dealer obtained filter. The noise that I hear is simalar to the noise that one hears after changing the oil the happens until the oil filter is filled. I suspected that the car had the wrong filter until I changed it & found that the old filter was a MB filter also. Does anyone have any experiance with this situation? Is there a check valve that is not functioning?