The way floating disks are made I don't believe they're going to fall off unless there are a bunch of the retainers missing? This noise could very much a disk problem but it depends on how much slop there is on either disk that it could actually not run true as the wheel runs true. The disk may actually be running so out of true that when you activate the front brake this snatches the disk and brings it back into a true running position, this may be causing the CLUNK you hear?
I would definately check to see if there are any of the retainers missing, if there are, how many? If there are a few of these retainers missing, I would replace the rotor assy. I would also pull the brake callipers apart and clean them just for general housekeeping. This will allow you to remove each part and check it out as you clean it, it will also allow you to examine each bolt to see if anything is missing or loose. With front end nioses one can never be exactly sure what it is until you find the actual problem. You say when you hit the front brake this noise stops, I firmly believe this is an excellent place to start checking.
To give you an example; I had a very bad metal to metal sound coming from the front of a rotor when the brakes were applied. I took everything apart to see what I was going to need to replace, I knew this was going to be expensive with this type of noise in the front end. I eventually found a stone had gotten wedged between the brake pad, the rotor, and the caliper. I could not begin to tell anyone how this large stone got into such a small place, but it did. After removing the stone and putting everything back together, I found I needed no parts, and the problem was fixed. When working on something like this, keep your eyes open and your senses about you, most of the time you'll end up finding the problem.
Just remember, even a blind chicken gets a worm every once in a while! I've found my share of them.
Good Luck,
clutchless1
