Good luck trying to diagnose any strange noise on a diesel.
A few weeks ago I heard an unusual sound from my W203 C220cdi and being a diesel, with different noises at different temperatures, I wondered if it could be the serpentine belt tensioner or one of the idler pulleys starting their goodbyes. The noise kind of disappeared and although I do not believe is self-healing engines, I did not give it more thought. A few days ago however, whilst doing a parking maneuvre, I suddenly had no power steering. There were no warning lights that would have indicated a broken belt. Then power steering was back and a "1 malfuntion" warning came up. Opening the bonnet, I was met with a mess at the bottom of the engine bay and immediately assumed a burst hydraulic power steering hose. The belt was intact.
Starting the engine I noticed slipping pulleys because of the saturated belt. I decided to drive the four kilometres home, risking a powersteering pump running out of oil. En route I had no powersteering issues but when I reversed the car up the ramp into the garage a low coolant level warning came up. Guess what? The waterpump had failed and it was coolant saturating the belt and messing up the engine bay, not power steering oil. To this day the car has never lost any coolant. The wear on the waterpump shaft however is excessive and such that the pulley and shaft wobbles and the impeller had eaten away the aluminium pump housing. The pump seemed to have failed in one fell swoop causing serious coolant loss past the shaft. Apart from a new water pump (~UKP 170) I have also ordered one (bargain price) idler (~UKP 7) from the agents as I could feel some grittiness on the old idler's bearing. After ~270,000 kms I am not complaining.
May I suggest that you remove the serpentine belt and feel the water pump, idlers, alternator, steering pump and compressor pulleys for posssible wear, or roughness.
