It is located on pass. side under the alternater.If so, this is what is going on.After starting a cold car, a clutch kicks in and this pump makes a lot of noise.After about 10 seconds the clutch kicks out and the noise goes away.
1.What is causing the noise?
2.Does the clutch only kick in once,because after driving i do not here the noise any more.
3.What is it's purpose?

I'll give you the fast & simple version.
When you cold-start the engine, a little extra fuel is provided by the engine computer.
This extra fuel, coupled with a little shot of air from the smog pump, heats up the catalytic converter very quickly, which minimizes exhaust emissions.
The pump will only engage on a relatively cold engine, and after that is inactive.

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2007 C 230 Sport.

Thanks for the info.Any idea what is causing the noise?

If the noise is excessive, you can have somebody that's familiar with airpumps, diagnose the problem accurately.
Given the age of your vehicle, I would'nt be surprised if it is going/gone.

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2007 C 230 Sport.

Someone on this site thought it might be the clutch bearing and suggested i call The Expediter.This guy makes Smog Pump bearings for 95.00 ea. I don't want to spend the mony if that is not the problem.

The typical solution is to cut the belt, and disconnect the wire going to the pump, as replacement units are expensive. It normally helps emissions during warmup, and its not used at all after that.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma