I hear very loud rumblings every once in a while. It seems to be normal as long as it's not all the time. I would keep an eye out for loose, soft, watery manure though. And I agree, add the grain slowly, a little at first, then slowly (like once a week) increase the qty.
Also, whey was he last de-wormed? Heavy worm loads can cause extra noise, at least it did in my girl when I first got her.