I have to agree with 'Ericnguyen' when he wrote:

It looks like that the automatic belt tensioner on your c240 is going south. When you hear a ticking sound at idle that would go away at higher rpms, the problem usually points to a bad belt tensioner (especially on newer MB cars).

I had my C220 diagnosed just today with that exact problem and symptoms. Unfortunately it's not all that inexpensive to correct as there are actually three parts to replace (at roughly $120 in parts and $200 in labor. If a DIY'r then it wouldn't be too bad.

good luck

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold