Whenever my pads get worn to the half way point, I begin to hear a grinding sound from the left brake assy(sounds/feels like front , but may be rear ). I inspect and there is no sign of scraping. What could this be? I always replace the pads prematurely so that I dont have to hear the grinding. In fact I replaced the fronts in March(rear pads/rotors last year sometime) and now the grinding has returned. The rears rotors were NOT smooth when replaced. They were "scored" as if in need of turning. I believe that a turning would probably correct the problem, but its my understanding that MB does not recommend turning these rotors. Any fixes anyone

On some non-MB cars I've owned, I've noticed over time that brake dust accumulation will cause a noise such as the one you describe. You might consider pulling the pads; then cleaning everything. Obviously there could be other issues.
What I'm suggesting is quick, simple; cheap.

As far as turning MB rotors goes, I think M.B.DOC had the final word on this issue. Check out what he said in the following post.

http://www.peachparts.com/shopforum/showthread.php?threadid=12589