It's definitely coming from behind the nearside rear passenger seat but that could be under the car then vibrating or echoing into the hatch. It could be the shock absorber. I only had the springs replaced. The rest of the strut is original.
Wondering why only the Spring and not the shock? Did your mechanic recommend that or against that? If it was your mechanics recommendation and the noise is the shock then maybe you can get a discount from the mechanic and save some money.
In the UK we generally only change shocks if there's a visible fluid leak. The springs were £75 each to change. I also had a front shock changed for £90. This got me through the inspection. Really 4 new strut assembly's would have been ideal. It needs other things though and costs add up. It needs two new tires soon and new brake discs on the front. The brake actuator recharges every third press.