I got the rocker covers removed today from the parts car that I'm cutting up. This thing has been sitting for years so any buildup behind the rockers is old. We had snow a couple of days ago and when it melted the moisture seeped into all that built-up crud. So when you drive your teener you put all that crap in there. When it sits (or you drive it in the rain) all that crap gets soaked and causes RU$T!
The moral is that if you haven't removed your rocker covers recently to clean this out, then you are abusing your precious teener.