Stiff suspension and bushings transmit more vibrational energy into the chassis exciting areas that are in the same resonate zone creating a great deal of noise, mostly lower frequencies, simple physics.
Hard tires with little sidewall flex add to this.
Hatch cars are a resonate chamber.
Cabin vents in rear corners need to be there but allow a great deal or noise in.
I know how to cure this to a great deal without adding a ton of mass, some mass yes, not a great deal if done carefully, the nost used product will be an acoustical thermal foam, the thermal parts is a free addition taking load off the HVAC depending on the weather.
I need to find the best way to let you search for the most comprehensive and proven method guide on the net, with low weight, low cost, real world results used in tens of thousands of vehicles around the world, and not violate any rules on the forum here as I am now retired from that industry and just here to help my fellow FiST owners
When I get to doing my own car I will post up a vehicle specific thread on it with plenty of pics.