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First, answering Emilime75, I don't know what are you complaining about, since the picture is clearly from a post-facelift, probably MY 2002 BMW e39.
The valve in question is called Secondary Air Injection Valve. The BMW part number is 11727540468
This is the EGR valve. It is pneumatically actuated and so, when starting, it should make some hissing sounds if it is still emptying the air in the accumulator. After the vaccum pressure is achieved, it should really stop.
If i recall correctly, the EGR opens in the high load area of the engine map, so that the temperature in the combustion chamber is reduced, say, from 750C to 400C. This happens because the EGR lets exhaust gases in the combustion chamber and so, reduces oxygen concentration. The reduced temperature reduces the formation of poisonous NO2 (the Nitrogen from the air ends up oxidized in high temperature).
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