Depends on the sound.
Does it sound like a valve tap? its a defined, rather loud tapping that increases speed with acceleration/engine revs. Valve tap indicates two things. Valves out of adjustment, but your car has VANOS, so this cant really be the case unless your vanos is failing. Check the valve cover gasket.
Where is it coming from, try to pinpoint this. If its coming from the oil pan, it could indicate a leak/failing oil pump.
If its coming from the valve covers, its a valve issue. check your spark plugs for oil. If the oil is apparent around the "top" of the plug, then you need a valve cover seal. I'm in the process of this.
Fan clutch is another suspect. When the car is off, spin the engine fan, it should spin with a little resistance. If its "freewheeling" your fan clutch is bad, if its difficult to spin, fan clutch is bad.
Also look for something hitting your fan.
Hope it helps!
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