Solving exhaust drone in hatch and wagon

[Gripless]

TLDR;

Drone isn’t like road noise normal sound deadening methods don’t work well.
It’s mostly about the roof and the gap between the headliner. Stuffing that with something foam or felt reduces the noise. 
You should notice that drone isn’t all around you and quieter. Exhaust now comes clearly from very rear of car. 
 

 

What I did that worked
 

Stuffing the gap and adding sound deadening to the roof skin makes a significant difference to both exhaust drone and muddy subwoofers. 
 

What you must do is put something on the roof skin before adding so if it ever gets damp it won’t hold water next to the metal. both sound deadening or the below thermal sheet solve this

 

You don’t need to add heavy tar/rubber sheets but can, though that likely improve rain and hail noise the most. 
 

If your car is a dark colour then you should cover the roof internal structure and braces at the top of windscreen. This is thin and for the top of windscreen area where you cannot place more than 2-3mm is about the only option. 
Any stick on fibreglass and alloy heat wrap works. This can act as your water barrier as well. I only used this over the rails structure. And front as I didn’t know the thickness of gap around rails. 
 

Front to back between rails structure closed cell foam 10-15mm thick was used. This is about the height of the rails so makes a level surface. 
 

What you want is Subwoofer ‘wool’ or ‘cotton’ which is plastic and often water repellent. The most common is PET. For other materials check fire retardant properties. PET melts but does self extinguish and the headliner is partly made of this. It comes in 30-40mm thick 550mm rolls. 
 

There is likely already felt pads glued to headliner. Remove these as this is what you are replacing with much more coverage. 

 

Depending on the model where to put the stuffing varies. But the general idea is to cover as much of the headliner as possible. Avoid or watch out for the following areas:

- windscreen upper frame as the lining almost touches it so no clearance 

- Around the lights. I just puts heat wrap around this and on to foam to seal them away from foam. Maybe a good time to swap to LED bulbs. 
- main rail structures. The 5-10cm wide ones where lights mount. The longing may curve in these areas to close the gap for mounting or clips. 
- anywhere the headliner curves upward 
- sides if you have airbags. Look to stop where the headliner starts to curve down.  Airbags blow out that area when they come down. 
 

I taped these roughy in place for testing to make sure the headliner still when back in and wasn’t pushed out anywhere. 
Once fitting correctly the stuffing was glued in place with a few squiggles or small 30mm circles of shoe glue at corners, sides and few middle points.