dai

if it's a core plug hole
yes a core plug,from the size that's what it sounds like but they are usually on the side
they rust out and leak,but if one had been replaced and not fitted properly,the new one could have just blown out
you push one in until it holds,then get a socket that fits the rim and tap it into place
the problem occurs when it's not tapped in squarely
if you have a friend that works on cars ect. ask them to have a look and confirm what it is