No, certainly not common. It may be more prominent if you have a top mount 'cooler with waterspray dribbling water into the bellhousing, or if the guard on the back of the underside of the engine has been left off and rain was sprayed up in there. A clutchplate material will also affect it.

When parking your car up for an extended period, just ensure it's warmed up and preferably dry when you leave it. Worst thing you can do is wash your car then store it.