Rear Disc Brake Thumping/Knocking noise

[gallat1]

Looking for some help here on a 07 A4. I just replaced the brakes and rotors all around. Everything went well except for the rear driver side. When I drove it around to break in the pads I hear a loud thumping/knocking sound from the rear left. After further inspection I saw that the other 3 rotors are nicely breaking in, but the rear left, where the noise is coming from is still untouched. It looks as though the pads are not grabbing. Any advice?
I got back, jack the car up again and check everything, even released the piston caliper, and re-compressed it, basically redid the rear left brake. Went around the block again, and again the knocking sound was back. Please help!

[gallat1]

Posting an update....well after several checks and retracing my steps I went out an bought a new set of pads to replace the brand new Hawk Ceramics. It turned out the Hawks were slightly under sized and each time the brakes were applied the pads would move just slightly in the housing to lay unevenly. So the edge of the pad was pinching, making a knocking sound. I could tell when I removed the pad. Maybe this helps others.