Chain or valves?

[Sid The Kid]

Hi, I have a 1991 500SL with about 116K km's on it. When the engine is warm i hear rattling, sounds like a marble rattling in a tin can. The engine is very quiet so this noise really stands out. Everybody points it out to me. The noise comes when the engine is warm and at every stop light or stop sign or idling in traffic it is constantly making this rattling sound. However the noise goes away once the rpms increase. if i put the car in neutral at a light and rev the engine a little there will be no sound. I usually do this to avoid the embarassing noise. I feel that this noise has come after switching to 15W50 oil from 5W40. Could this be the reason? with 116K i dont think it could be the chain? I know the previous owner once went 30K with no oil change and another 20K with no oil change before selling the car. So maybe it is sticking valves? but i thought valves would make more noise when cold, not warm?