There are three covers (for a baby restraint/harness) on the parcel shelf, these "click in" but still move around a lot.
I placed some foam underneath the caps to suppress the noise (the foam kept a small amount of pressure on the covers, preventing them from moving around as much).

I'm actually tempted to get some superglue or silicone and seal off those covers, on a trip down south recently, two of those covers managed to come undone and go flying in the cabin (was a very bumpy road).

There is a lot of people on NASIOC who have been complaining about noises throughout the entire cabin area and there are a number of solutions posted.

You could also look at putting some extra foam lining or dynamat? under the rear parcel shelf.
Essentially anything that helps to lockdown the movement in the shelf, or absorb noise and vibrations should help out.

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