If you have access to a smoke machine, I prefer to hook it up and confirm that there will be smoke coming from under the plenum.
One time I found the leak was in the pump itself.
Once you see smoke coming from under the plenum, you will now prepare for your first plenum pull.
Likely a hose has fallen off.
Marc's plenum pull Instructional or just look in the stickies on this forum and you will find any questions answered.
Order your plenum gaskets from Jerry's first.
Keep us posted, nothing really complex about it.
Marty