This little guy.

Once you remove the two screws, you will see a round part with two nothces like in Fig. 1. Find a way to get something in those (I used part of a broken hacksaw blade) and tighten it as much as possible. Try to use a new rubber O-ring under the cover.

If that doesn't work, try another one. It should only be about $10 from a junkyard.

This, assuming you've never had the throttle body or upper half of the intake manifold off. If you have, it may be a vacuum leak.