good idea
So, I just tried that fix. Mine was rattling like a mo-fo. I tried moving the hook up, but it made it worse. I moved it down, toward the floor, and it worked well. I know write-ups have been done in the past using this method, but I thought I'd update it for us newer guys.
I also went to the local hardware store and bought some faucet washers. A single six-pack cost me $1. They are 1/8" deep, and 3/4" in diameter. You can see from the pic below where I put them. Two on each stopper. Works wonders. (The one under the stopper is just in place with the pressure from the bolt that attaches the stopper to the frame. I doubt it will dislodge. So, now no rattle. In fact, when I open the lid right now, it springs up a little, so I'm going to have to move the hook back up a little I think. All in all, easy fix.
What's that strap you see in the background? Well, I got tired of my sub sliding around, so I went a little overboard and now this thing doesn't move for anything: