On he left side of the hood, a bit behind the cab. This is a standard feature on EMD hood-unit designs from the 1960s (GP30 or GP35) on. Originally with a straight vertical side, then (early 80s?) revised to fan outwards like this for freer air flow: reducing power needed for blower, and so slightly contributing to fuel economy.
Often called "turbo bulge" in railfan sources: it's not connected to the turbocharger (as witness the fact that non-turbocharged units like the GP-38 have it), but it resembles a similar bulge on the hoods of some Baldwin roadswitchers which may have been to accommodate the turbocharger.