I find it difficult to judge whether "rumbling" is happy or not. So I tend to also look at body language. If he is stroked on his head or behind his ears, my Hazelnut makes what sounds like a deep rumble and tries to shake your finger/hand off, so I am of the opinion he does not like it. On the other hand when his back is being stroked, he lays very flat, splays his back legs out and makes a sort of chortling sound, so I judge he likes that.