Strange Sound while in reverse

Lately, I've noticed a strange sound coming from the rear of the X whenever I'm backing up. It sounds almost as if the park brake is still engaged, and when I gently apply the brakes, I can faintly detect a light grinding sound. I did a forum search earlier this morning, and I think there was a member with an e46 that was doing the same thing. Someone mentioned the "park brake shoes/pads", but there wasn't a conclusion. Has anyone had any experience with this?

In addition, after going on a long drive, I sometimes smell the aroma of a burning substance (oil/fluid) whenever I walk toward the rear of the car.

Best go have the brakes checked.
Break sensors (the dash lites up) are only on the front right, and left rear wheels. Could have a pinch in the hand brake cable that is keeping some pressure on one of the rear wheels.

If it is an issue with the Handbrake, keep in mind that the Owner's Manual says:

To prevent corrosion and one-sided braking action, occasionally apply the handbrake lightly when the vehicle is slowly coming to a standstill at a traffic light if the traffic conditions are suitable.

Particularly applicable to areas where salt is used in winter.

"If traffic conditions are suitable" means of course that anyone following you will not see any brake lights coming on if just the handbrake is used.

Thanks for the input! Ironically, the noise just ceased yesterday, and I cannot replicate it. I'm actually planning on replacing the brakes soon anyway.