'Rattle' on starting

[cornishbloke]

I have a 2013 passat estate highline 2.0 TDI Blue Motion. This has happened about 3 times now, on either a wet or damp morning when its raining or has been raining over night. When I start car first thing in morning, there has been what sounds like a 'rattle' coming from the engine bay on the drivers side where the belts are. The 'rattle' ony lasts for about 2-3 seconds, then all is OK. Turning engine on then off there is no noise.

This has only happen about 3 times, is it because the belts are damp? Anyone with any ideas?

Thanks