Hi, I think my freshly acquired 500 has the same. Sounds like a diesel, esp.when warm. It's hydraulic tappets from what I learned. You need to check them..I'm going to open the engine next week, bought new seals today to be ready.

 

Quite possibly a sticky lifter as above. You can try using oil flush next time you change the oil. There is a hydraulic lifter additive available which can help too.

 

I have the same problem with my '91 560SEC.When starting the car, it sounds like a loud clanky clank coming from the front of the engine eventually goes away when driving.

I diagnosed the problem and discovered it was the radiator fan hitting the fan shroud. You might want to move the fan shroud a little to see if the clanking stops. From what I've read, this problem arises from bad motor mounts. As the mounts deteriorate, the fan lowers and hits the sides of the shroud.

 

This is something else..however motor mounts should be changed every 50.000Kms if you do a lot of city driving.

 

There are tow things that may cause this sound. If the sound is too loud like tuck...tuck, I think this is due to the rocker arm. If the sound like tick...tick, I may accuse the hydraulic lifer.

Here what I faced , my problem on my 91 was ticking noise on one side of the engine, I opened valve cover, removed all the hydraulic lifters, inspected them and found two of them were partially stocked : they were not moving the whole travel, they were not expanding as they should be by the spring force on them which makes a gap between them and the rocker arm " cleaned them using cleaning agent, put back and they were fine. No more tick tick .... sound.

Here is an example of a bad rocker arm.
Goodluck

 

Thanks for the explanation. What mileage had yours when it happened? How hard is the job and what you need to have prepared new except cover seals?

 

super quick fix, it was the radiator fan hitting the radiator. he just lowered the radiator. nice...
The motor mounts were done a month back maybe something was misaligned after this.