Well Leah, it could be a number of things. I happen to have the same problem on my 99' V6, and have done quite a bit of research here. Here's what I've discovered might make that noise out of the rear:
1.) Tires that are worn down un-evenly. However, if your tires are new (and balanced), then that likely isn't the case.
2.) A rear wheel bearing. This is what your axle rotates around in the wheel hub assembly. If this starts to break down, it could cause a whirring noise, as well as differential fluid leakage out of that wheel well area at higher speeds. I have a feeling (from what I know about the noise, which is limited) that this might be the case, if one of your wheels was jammed really hard on one side. However, it could also be
3.) Something with your differential/rear end. When people mention your rear end (and I mean that in the most civil way possible), they are referencing the entire differential, gears, axles, and rear wheel assemblies, so when one person says differential and the other says rear end, they are essentially talking about the same area of the car. When your car got hit (and I don't know how badly it got hit), it could have jammed something and bent something. Unfortunately, it could have been your axles, your spider gears (that are inside the differential), the ring and pinion gear (where the drive shaft meets the spider gears), or the rind and pinion gear seal.
So, on to diagnoses. My noise happens to be a ring and pinion whir, and happens whenever I step on the gas at higher speeds (usually happens the loudest between the speeds of 40-60), but goes quiet shortly after letting off of the gas and I begin to coast. Does this sound familiar, or is it a constant noise?