Stomach noises are good
I have had horses in the past that if you couldn't hear their stomach from across the stall you got the vet out immediantly, so it's not unheard of to have a "loud" horse. You should keep an ear out and see if this is normal for him as abnormally loud gut sounds aren't necessarily good.
As for the grain and the hay...what were they? Are they the same stuff the previous owners were feeding him? Feed changes should be gradual as abrupt changes can cause problems although I suppose it's a bit too late to be telling you that now.