That would be a bit worrying to me. If you're not playing anything while tilting it that way, then I don't see how the speaker crackling problem (which the last patch was supposed to fix already, I think?) could be related to it. And since everything is soldered inside the machine, there aren't any loose parts so there shouldn't be any sound coming from something inside it when tilting it any way.
Personally I'd bring it into an Apple Store or call AppleCare hotlines and have it checked out. It could be a sign of a much bigger issue, so I wouldn't take any risks. Maybe the fans aren't mounted correctly and are hitting something when you hold it in a certain angle (which might explain the regular rhythm) or something of the like.