M104 3.2 piston rattle?
 

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.

That engine has a relatively short piston skirt and is prone to rattle like you describe. I've never seen any ill effects from it though.

Piston rattle does not sound good. ;):D

Glad I have an M103. :P

M 104 engine is bulletproof never heard of any engine failure. Could it be the chain tensioner?

hyd. lifters sometimes make a noise

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.

I'd bet money its piston rattle, I've seen a bunch of them that do it. On the 103 the piston skirt is long enough it protrudes about 10-12mm below the bottom of the bore. The 104 piston's skirt is short enough it doesn't protrude below the bottom of the bore at all. That short skirt allows the 104 piston to rock in the bore especially when the engine is cold and the clearances are larger. I wouldn't worry about it, unless you plan to run it near the redline a lot when it is cold.

mine used to rattle ,until I switch to 15w40 diesel oil.The additives in diesel oil,must have done something

A synthetic or heavier oil may produce a stronger persistent film on the cylinder walls minimizing the issue....maybe?