As Mike says and when you have decided everything that should be tight is so, search for "SeaScoobys" excellent article on soundproofing/deadening. I have around 70lbs of the deadening on my car, which is overkill, but it is substantially quieter than when I got it. The SF series were very poorly insulated but the later models got progressively better however, they still aren't on a par with a modern saloon car in my opinion.
The worst offenders are the panels at the back of the car, the trunk and rear quarter panels (over the mudguards) along with area around the spare wheel well. Applying sound deadening to these panels increases the mass thereby reducing/lowering the resonant frequency to a point where it's no longer annoying, sending it down there with tyre roar and transmission sub bass. Anything below about 40Hz is acceptable to most folks but if you have big fat tyres, then the 40-120Hz region is dominated by their 'roar' hence the propensity for installation of sub woofers in modern cars/cars with wider tyres, to claw that low bass back.
Carefully pull the trim on the side panels in the trunk and lay some sound deadening on the outer skin, tapping on the outside till you are happy with the "thunk". Next you need to stuff the rear quarter panel space really tight with something absorbent, SS recommended the filling from really cheap and nasty (do I hear Walmart?) pillows and I can vouch that it is economical and works. At least one pillow per side and at probably more than one for the trunk lid cavities (I'd buy at least 4-6 pillows)
Pull the trim from the inside of the trunk door and again, lay some sound deadening down using the tap method on the outer skin. Once that is done stuff the remaining space with the hollow fiber but making sure you don't foul the pathways for the locking and wiper mechanisms. Apply some sponge to the back of the main trim plastic panel as well because that's hollow too. Careful with the trunk lid though because if you go too far (I mean really too far) the struts may have trouble supporting the extra weight. I have loads of deadening and stuffing and the lid needs assistance to lift (just a little) but stays securely where it is at the top, it also closes nicely without needing to be banged.
Lift the rear flooring and remove all the ancillaries under it. Line the floor space with sound deadening and fill any obvious spaces with stuffing.
As a rule on applying sound deadening, tap the panels and if they sound hollow apply the matting. If the metal already sounds solid, you are probably wasting expensive materials. I did as far as under the rear seats and some big square panels under the carpets where the feet would lie.
Subarus have rather thin sheet steel on the older models, not sure about the newer ones but pulling the door cards and protective film will allow you access to the inner and outer skin. Some folks just plaster the inner skin and leave the outside one, which doesn't make a lot of sense to me. Again, use the tapping method to determine how much the panels need but be aware that this material is quite heavy and if you go overboard (complete coverage, doubled), the door hinges will eventually start to sag.
If you can release any of the other trim panels that don't have moving parts behind them, like the trunk trim around the side windows, hollow fiber can be stuffed in there as well but be careful about jamming up the fixing clips with it.
I did the under side of my hood too but couldn't say whether that has been a success or not. Some guys will say no way you should be adding all that weight but in the overall scheme of things, sensibly and judiciously applied sound deadening would equal a weekly shop or the crap a lot of folks permanently carry in their trunks anyway.
It is also worth bearing in mind that the Forester is sitting on a permanently active AWD (unlike many so called "AWD" cars that are mostly 2WD), often with larger tyres fitted, and that noise will be transmitted into the car. This was apparent for those of us who as children found the back of the family car uncomfortable and noisy, but when the majority of cars moved to FWD from RWD in the late 1970s, all of a sudden, the back seats were the place to be now the rumbling prop shaft and diff were gone. Comparing this scenario to something like Jaguar or BMW though requires you to go and lift the floor insulation on one of these cars and see that it's a big deadening mat, usually at least 2.5" thick (Jaguar anyway).
And a small tip when applying sound deadening (like Raamat or equivalent), wear heavy gardening type gloves and use a ball ended hammer, around 3-5 pounds, for pushing the adhesive matting into the panel mouldings, like the trunk floor. As most of these mats have a bitumen backing with a thin aluminium type front, your bare hands will be cut and marked quite badly without you realising.
