The other day I started my car that had been parked for about 30 minutes and had the DS door still open. I heard a noise from under and forward of the door.
It sounded like a "clack, clack, clack" and it lasted for about 15 seconds and quietly went away. I went home, put the car on the ramps and had my wife start the car, but hear nothing. This morning I heard it again and with the hood open, could hear it from the DS low on the block when I ran around to the engine. Again, it went away within about 20 seconds or so. I have a 5 bar pressure gauge and it is reading normal for all ranges of rpms. I've changed the oil every 2500 miles without fail for the last 418K miles. Could it be bearings deep in the engine or a failing oil pump or something else?

Thanks for your insight.