Are you talking about the hole at the back end of the lower intake dead center against the firewall that should have a rubber grommet in it. If so its the PCV. If you plug it make sure you have breather caps on your valve covers. If it isnt that hole, then is it a threaded hole? If so it should be the EGR which is fine to plug.
its an exaust cross over port like mike said its egr junk the way you plugged it is good as long as the plug is recessed into the head enough. you can also get intake gaskets that will actually come with a piece to block it but what you did is good so dont worry bout it your all set
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