Are M 104 motors known for piston rattle? Now that the weather is colder, my 97 SL320 with 125K has a slight noise when cold just before a light throttle shift or if I float the motor. I've heard motors make a similar noise when the rod bearings are slightly worn but this motor is silent when started cold after sitting overnight. ( Loose rods will make some noise on a cold start if the motor was up to operating temp last run cycle. )

It is slight enough that a regular driver would not notice anything different.

I've built piles of motors over the years so I've got a good handle on what sounds worn parts can make and know that each motor has it's typical failure points.