Delayed Oil Pressure on Start

[jfhWY]

In the last few weeks when starting my car (1990 300TE 4MATIC) first thing in the morning (cold from -20 to 35 F) or after sitting for a while the oil pressure gauge takes about 5 - 15 sec to come up to pressure. This morning I had the door open and I heard the valves knocking until the pressure came up (yikes!). Otherwise, gauge & pressure appear fine. Car runs fine, starts fine, regular oil changes etc. 10w30 oil and Mann filter. Have done some research here, wondering if this could be my oil pressure sending unit or possibly oil pump. Also, I'm planning to install a block heater soon. Any ideas greatly appreciated. Thanks!