my solution was 4 things:

1. mine is a folding rack and the hinge was loose and eventually the springs broke. I removed the springs and wrapped thick cable ties around and pulled them as hard as I could. see red arrow in picture to illustrate rattle movement and placement of the cable ties - took a few goes to come up with an effective way to wrap the cable tie.

2. used a strip of door / window seal from the hardware store around the _inside_ of the rim to make the lid snug when closed. had also experimented with doing it around the rim of the upper half but that didn't work well for me. I also tried a few different seals such as a soft foam but this brush style one worked best, the foam ones stopped working when the foam got old.

3. wrapped some electrical tap around the lock hook a couple of times. see yellow tape in last pic

4. stuck a firm foam pad between the box and the rack

the above steps has 100% cured it for me no matter how bad the road is.

plus this is also handy: GTS box liner

I never got around to trimming the cable tie, I just tucked it under the box

foam pad. ignore the cable that's for my extra brake light, I might run that inside the case sometime