Noisy floating wall anchors
After a lot of time wandering the faster aisle at my local Ace, I ended up finding a rubber grommet that fit snugly around the 6d nails I am using. I then took the grommet and found a fender washer that fit the grommet inside it (ended up being a 1/2" washer).



Then I drilled a hole in the floating bottom plate large enough for the grommet to drop into (about 5/8"), so the washer would sit flush against the top of the 2x4. I set the washer/grommet in there and pre-drilled 2 holes through the washer. Finally I screwed the washer down with 2 pan head screws to hold it in place. Before I drywall over everything I'll spray the washers with some Armor-All just for grins.



I haven't had any problems with noise with this approach, and the wall doesn't seem to move any more than it did when it was spiked the normal way. I also stiffened up the entire wall assembly by toe-screwing every stud into the top and bottom plate with some heavy-duty SPAX screws, so that the nails didn't work their way loose.

In regards to Michael Karas' suggestions, stiffening the floor seemed like overkill. I've never felt movement in the floor from above, and even having a 175-lb. person jumping on the floor upstairs I couldn't see movement in the floating walls.