When to change out timing chain?
 

I have a 1990 300SE with about 190,000 miles on it runs like a champ that god. Im worried about when I need to replace the chain. Is there a sound I should look out for (warning sign). After I get home from driving and I put it in park I here like a diesel type of sound when in park is that the chain? And where is the transmission dashpot located?

It's probably time
 

Sometimes there's a rattling sound you hear on start up, and then it quiets down after awhile.

I'd suspect you are due. I know for the 8 cylinder engines they recommend chabging out the timing chain, tensioner and plastic guide rails every 100,000 miles or so...

These cars are very reliable, and the 6 cylinder engine is VERY robust, but by now you could be tempting fate, if the chain slips, and lets go, the pistons could collide with.....well, we are talking about a major catastrophe, let's not go there, and by now especially the guide rails, are probably pretty brittle, and maybe about to crumble, so go ahead and plan to replace the chain tensioner and guide rails, so as to be on the safe side.

The Indie mechanic I like had an '83 380SL bought in with a major engine failure because the chain had slipped. What makes it a story was that the car had been brought in for the timing chain to be changed, and it was, but the mechanic did the job at another shop WITHOUT changing the guide rails, and the new chain slipped-again! So, the mechanic changed the chain again and AGAIN he did not replace the guide rails, and the timing chain slipped ANOTHER TIME, and this time the guide rails just crumbled completely and the damage was repeated yet again!!

At this point the customer switched mechanics and went to my Indie, and I was there at this point to see the poor car with the engine torn down for the 3rd attempt. My indie knew what had happened and why, But the customer didn't realize.

your rattling sound may be the belt tensioner shock. pretty common. press down on the shock when you hear the noise and see if it goes away.