Hi everyone... I have a 2012 WRX sedan and noticed a bad vibration from the rear shelf as well. After tapping around everywhere and getting the vibration noise, I finally put a light pressure on the tab for what the OP has described as the "keyless module" where it sticks into the trunk and the vibration stopped. I jammed a small piece of automotive carpet into the hole next to the tab and it seems to have stopped the vibration -- the location is on the left side of the trunk about 8 inches to the right of the large square opening with the insulation in it, as shown in the attached photo. Not sure if this is a permanent fix, but it only took a few seconds to do compared to pulling the whole shelf apart, so might be something for others to try first.