the dipstick seals tend to wear out sometimes. I'd try O-ringing it. It happens because the intake is connected to the valve cover, which shares the same air as the dipstick tube. Under vacuum, it would suck the dipstick back in.

There might be something with the PCV, aswell. I don't know very much about how they work, but I know it has something to do with venting the crankcase.