simple yet vexing 300d question

[acl4]

A few weeks ago I noticed a new noise that seemed to be coming from the turbo of my '84 300d (a whining chirp when the engine is under heavy throttle on the road). Until now the turbo never made any noise. Looking around under the hood I noticed that there was a threaded hole on the rear of the intake manifold (passenger side, just forward of the banjo bolt) that had less crud on it than other parts. Sure enough, when running, air pumped right out this hole (about 1/4 inch in diameter). Question is: what should be in this hole? Can I just plug it with a bolt or am I missing something important? BTW- car seems to run pretty well without anything in the hole, though I still want to fix it!

I have included a pic to help identify it.

Thanks,

Andrew