My DD did the same thing for a while ... from about 4 months through to 6/7 months.

Initally I was worried, like you, and then was told by several people (medical people included) that babies often make grunting noises just because they can. As adults we think the constant making of the noise would hurt their little throats, but no, it appears to be a really big game with the under ones!

When DD got the noise going EVERYONE would say 'Oh I think she's doing a poo' ... and we'd run in and change the DRY nappy.
I got that sick and tired of telling people 'Oh no, it's just a noise she likes making' I started to agree with them and say 'Oh yeah she might be doing a poo, can you change her for me' ... once they'd changed several DRY nappies they soon stopped commenting on the sound.

I guess it's just another part of odd baby behaviour - but yes, they do grow out of it.
DD is now almost 10 months and no more grunting.