M103 / M104 have the same block / lower end. M103 has single cam , M104 is twin cam.
And, there is the "twin cam " M103 found in the 90-93 ish 300SL

Now that I'm getting more cold driving time, it is definitely piston rattle. ( 302 Ford motors did this a lot as do late 90's Gm. )

Lifter noise won't change volume with engine load like piston rattle will.