It's parked either on the driveway or on a large patio round the back of my house. Both areas almost horizontal, and it's been parked both ways round and it does it randomly i.e. no correlation with temperature or the amount of time it's been standing.
First time it ever did the rattle was about 4 years ago when the oil level was at the minimum. There was a time when it was doing it quite a lot but not as much now. I use Castrol 0w30. The fact that it never used to do it and now does suggests a clogged oil-way or a problem with the pump, that may not require any attention but you never know.