Woo-hoo-hoo!
As you've doubtless already noticed, when you do a huge jump in the Warthog one of the Marines is likely to give an exuberant whoop of some sort (if he hasn't done one recently). There are some good jumps in the area where you get the Warthog; it's a great place for just bombing around having fun.
Big mound in the valley
There's also a great jump on the way to the rockslide area. If you've just emerged from the underground tunnel, and you curve around gently towards the nearest entrance to the rockslide area, there's a big mound which lets you jump for miles, right over to the entrance. Better launch level though, or you'll land badly and come in for some verbal flak. Talking about emerging from that underground tunnel, you can actually get a whoop from a smallish jump practically straight ahead if you want, close to the edge. There's room to land and swerve before you have a disaster.