Tap tap or metallic clank at start up on cold days

[HuskyMan]

It has turned off colder (around 40 degrees at night); when starting the 1991 560SEL first thing I hear a tap tap tap or metallic clank for a few seconds. Then, the car is quiet the rest of the day, both while driving and upon starting again after operating temperature is reached. About two years ago, it was making sounds upon startup so I switched to Chevron Delo LE 15W40 motor oil which quieted the tap tap tap sound.

This morning, with the car cold and as an experiment, I removed the ignition switch cap located on the fender wheel (driver's side) and turned the starter until I could see oil pressure rise. Then, I reattached the ignition switch cap and started the car. The car started immediately with no tap tap or clank whatsoever. Any ideas?