always happens mate after the cars been sat there for ages and once you've driven it for a few minutes it goes clear. Not sure why though im guessing its crap from your engine?
 
If it does it when the engine's warm, you've probably blown your head gasket. Check for "mayonaise" on your oil cap and check your coolant level daily.

If it only happens when cold, its probably excess water in the exhaust burning off.