Hissing noise please help

Hi guys just been going all around this range that I've bought off a mate,had it running on my drive to go through a few bits then when I switched it off a hear like an air hissing noise, I looked all round engine bay then I found it, in the expansion tank there is a tiny pin hole in the top right at the very back near the bulk head, it looks like its got a piece of rubber in there,
Should this hole be there as I can never recall one being fitted on the other ones I've Had in the past.