I've an interesting little thing going on with my 2007 B180 CDi which has a slipping belt somewhere in the engine bay. I've only heard it on wet days and whilst the engine is relatively cold. When slipping there is a slight effect on engine rpm, perhaps by 50/100 rpm drop which recovers once the slipping has stopped. It normally only happens once whilst driving away from home in the morning so replicating it would be really difficult. It's very weather dependant too, only happening after very heavy / persistent rain. Any assistance would be greatly appreciated. The car has 33500 miles on it and had a clear 'star' check in March with a dealer.