My 190e 2.3 also sounds like a loud diesel under the following conditions -
Under light load between 1,000 and 1,500 rpm , whether hot or cold (it makes no difference) i.e. in drive with brakes on but light accelerator will generate the most noise.
The sound definitely comes from the valve train and has been there since I purchased the car.
Recently I have had the Head rebuilt , new chain and tensioner and it has made no difference - possible a bit worse now !.
The rockers and camshaft are all o.k.. My mechanic says that if it was due to wear in the cam or rockers and the Hydraulic lifters could not adjust for the wear then it would be noisy all of the time.
I heard another 190e 2.3 last week take-off at the lights and it also made this horrible noise at low revs.
Any ideas ?