It could be you axle nut is a bit loose. I had a noise coming from the rear of my S last year when I backed up and it didn't always occur and it turned out to be a loose axle nut and this apparently is common on s2ks.

 

I'll try that... although, that is one of the things I already considered. The nut has not moved at all since installed. Normally if it's loose you'll hear some clicking noise upon accelerating. Not groaning while only in reverse... I'm completely puzzled.

 

FIGURED IT OUT! It was the right rear brake caliper. My co-driver and I recently replaced the rear hubs. When he did his side he didn't torque the bolts that bolt the slides of the caliper to the hub... one bolt was missing completely. So there you have it. Replaced the bolt, torqued everything, no more noise! YAY!