What brand of brake pads did you use? Did they appear to match the size and shape of the old ones? I'm partial to ATE brand. I found they actually sweep more of the rotor than whatever was in there before (they had an MB emblem on them)
I always suspect the most recent repairs as the cause of new noise.
You may be able to narrow down the noise by jacking up one side (please use jackstands and don't rely on the MB jack) and spinning the wheel as fast as you can by hand, and having an assistant step on the brakes sharply. From your description it appears to be the transition from forward to reverse, so spin the wheel first in one direction and then the other.
Being closer to the wheel may help you decide where the noise is coming from, if you can repeat it.
Next, if the noise is repeated by the above test, take off the tire and observe the brake components as you spin the rotor by hand and have the brakes applied. You may be able to spot something moving. You should have spring clips against the pads that are held in place by the pins. Are they missing? (Haynes manual calls them "anti-rattle" springs.)
I don't really think this test will necessarily find your problem, but its a start. Keep us posted on what you find.