It's likely the hatch as pointed out, and there are several ways to fix that:
1. Tighten the hatch down. Four 12mm nuts behind the dome light panel.
2. Unscrew the rubber stoppers on the hatch until they're only in about one half turn.
3. Wrap the U-latch in heavy gauge vinyl tape, it's more durable than electrical tape.
4. Remove the shim under the U-latch. Those screws are VERY tight though.
Other noises I can think of: If your car is lowered with an aftermarket exhaust, the piping can hit the lower control arm. Loose interior panels can also shake around a bit, but they will have a distinctive plasticky squeak.