2009 with intermittent Jake brake sound under car

There's not much going on back there with the rear brakes whether you have drum or disc depending on what country you're in still not much back there but a hose the literal brake parts themselves caliper wheel cylinders etc and that's about it if anything's bent or out of alignment or something like that it should be pretty easy to spot and realizing that on a generation 2 at least the brakes in the back are handled by like an electric pump so maybe they just have some air in them they can honk and make funny noises when they're not operating properly usually this has to do with either low fluid and door introduction of air.

 

Did the OP jack up the car and have a look around underneath?

There might be a plastic bag or other piece of debris stuck in there somewhere that intermittently shifts into a position where it can flap around.

Also check that the rear bumper cover, rear fender liners, and the like are still firmly attached. For the same reasons as the preceding.

When the car has cooled off shake the exhaust system, as that could be loose too.

 

loose aero cover?