My Left Knee Makes A Grinding Sound When I Walk

My left knee makes a grinding sound when I walk up the stairs. I go up one at a time. Three months ago, I was diagnosed with Chrondamalacia Patella in both knees. The right knee was the better knee so I walked up stairs with that leg first and go down with the left leg first. The last couple of days, I decided to rotate legs while going up the stairs (up with left and down with right) and that is when I noticed the grinding sound. That sound is new.
I have been doing the physical therapy they prescribed faithfully. Any suggestions?