I would be surprised if it was leaking there.
Where did you buy the keg?
Did it come pressurized?
If it didnt come holding pressure I would do the following
Some of my kegs are trickier than other to seat the lid. Perhaps you didnt do that properly.
FYI, hissing is preferable to not knowing where the leak is coming from, theoretically this should be an easy fix, unless it truly is coming from where you said it is.