Sounds like more than just moisture. Sounds like you have a coolant leak. Are you losing coolant?

 

your saying a coolant leak inside of the engine?

 

bump...

 

could be coolant. makes that coagulated tan looking gunk that sticks to your oil cap. might be a head gasket. its not sludge. pay attention to what your coolant level looks like. sounds like its not bad yet, but if you start seeing more, or smoking, or your coolant level starts dropping significantly fix it quick, because that will blow up your engine