Mercury Sable makes noise when turning left and backing in? It sounds like a grinding noise.

Sounds like a CV (Constant Velocity) joint. I can't completely solve this for you, but you can re-post with the CV problem. The sound - grinding and or popping - should be coming from only one side of the car - but it could be the inner or outer CV joint on that side. Check the rubber boots around the joints - they are located on each end of the drive shaft on the transaxle. If you have a torn boot, that will probably indicate which joint is bad.