To the OP, I've successfully made my dd 07 Focus as quiet as a MB!
Seriously, you need to attack the noise from OUTSIDE the cabin for the best results. It's cheaper, faster (no need to remove interior panels etc) and quieter than applying it inside.
The two products links are below. In total I spent about $30 in materials as well as some old household bathroom towels.
Rubberized Rockerguard Undercoating | Canadian Tire
Dominion Sure Seal Brushable Undercoating | Canadian Tire
The stuff in the spray cans I applied to all 4 wheel wells as well as anywhere there was exposed metal under the car. I used about 2-3 cans when on sale.
The goop stuff in the tin I applied to the underside of my hood since I don't have an insulation pad. It dried quickly and is very thick.
in the trunk area, I removed all 3 panels (one on each side covering the wheel wells, and the floor one where the spare tire lives). I then draped the towels in these locations. The thicker the towel, the more noise it'll absorb. Don't worry too much about securing it down. I'd recommend removing the spare (if you have one) and spreading out some towels there too. I only have one behind the rear seat, but will probably add more to that area in the near future. Dont' worry, once everything is back together, nobody but you will know it's back there
The result? FAR quieter than this car has ever been! I was in the process of buying an older lexus/Avalon but decided to do this instead just to see if it would work.
Downside?
I used to enjoy the noise from the stock exhaust when at WOT. It was raspy, but kinds sporty at the same time. Now I still hear the exhaust, but it's not as pronounced as before, and I don't have to wear ear plugs when driving on the highway!
Anymore q's? PM me