That crunch - ooooh, how I hate it. I'm trying to do a far more detailed battle against rust than previous owners did. This includes pecking around with a screwdriver in suspect areas.
Anyway, my car has some serious driver's side issues where the splash shields go over the side panels, firewall, and cowl. The cowl itself is in good shape so I was removing the old seam sealer to do a "recaulk". I noticed just inside the cowl drain opening that there was excess sealer and coating, so I started scraping it out of there. When the screwdriver got near the front edge of the cowl, just inside that drain opening, there was a sickening "crunch" and a new hole appeared which would let water drain directly into the passenger compartment. DOH! I felt and heard that sickening crunch before I saw the hole, and it turned my stomach. I knew what it meant, and it wasn't the first time.
Since I don't want to remove all my framerails to look inside, I plan to go after them and my rockers with a small vaccuum hose, then spray in some Eastwood Rust Converter followed by their Heavy Duty Antirust product. Meanwhile I'll be poking around and hope I don't hear any more of that sickening "crunch".