Welcome to the club!! 9 out of 10 satria owners will face this kinda problem, not excluding GTi owners also. There are few possible areas u most probably wanna check:
1. Boot lock, apply grease to get rid of the sound.
2. Rear passenger seat, side panel (where the speakers are) clips. This, will depend whether you wanna take the risk dismantling the whole side panel, apply grease to the clips and getting it back into the right position again. It's gonna be easy for those who already had experience in it but a hard time for beginners though.
3. Speaker board, the board hanging to your boot hatch.
4. Front passenger seat, the gears and joints tend to squeak cos always pull here pull there.
These are a few I can think of or remember so far and do note that there's no assurance that the sound will not occur again in the near future. Happy troubleshooting!!