I know my laptop speakers suck, but that sounds mechanical not exhaust related!!! Like Seachicken said, pulling valve covers are free. You can check for loose rockers, gaps in pushrods to rocker arms, broken valve spring, broken guide (or whatever the piece is the rockers sit on), messed up lock ect... How does the car run when the sound happens as you drive? Any miss, sputter, or still smooth running?

But I guess dropping an exhaust valve could be exhaust related...