Soundproofing on the cheap

This might be something: http://www.overclock.net/t/447164/cheap-insulation-mod#post5399092

Haven't done it myself but looking around for ideas as well.

I used this on my computer and love it. The price is great.

Cheap
Quiet
Fast

Pick any two.

The easiest way for a computer to be quiet is to make it stop producing noise. It is not as easy to dampen vibrational noise, and very difficult to absorb airborne noise.

Find out what is the noisiest part in your computer. Do what you need to do to make it quieter. Easy.

One way to greatly reduce noise is to reduce turbulence in the case. Fran grills, HDD cages etc. cause turbulence and thus noise.
Cutting out the grills will help a lot and not reduce airflow, it'll actually marginally improve it. Of course, it will decrease resale value for the case and void the warranty.
Removing HDD cages that aren't in use will help.

The stock fans on the H100 are pretty noisy from what I have heard as they were meant to show the maximum cooling performance of the unit. Might be worth looking into better fans if you have the money for it and willing to spend it. One word of advice on fans: Don't cheap out on them. I've bought many fans over the years because I want to save money, and they are just fans right? Well, it's cheaper to buy 1 set of good ones instead of 3 sets of bad ones (or fans that are too powerful like the Scythe GT AP30's I bought once). Better fans often come with rubber screws / gaskets to dampen the vibrations. Even though it's not that big of a deal, all the little bits help.

Additionally, more fans are louder, no matter how quiet they are on their own. I advise you to unplug a couple of them, see the effect on the temps and decide if you need them. I used to be fond of undervolting fans myself but a lot of the cheaper fans will introduce bearing noise / vibrations when undervolted.

Dampening the HDD also helps but the WD green is very silent as it is so I wouldn't really focus on that.

Reducing noise is a big step-by-step process where you constantly run into a different component that becomes the noisiest. As Zap said, deal with the source of the noise first.
Soundproofing material is more the icing on the cake, the final step.

flashing tape looks like a great idea for 20 bucks almost a no brainer. however, i used leftover dynamat. had a roll of 3' x 300' leftover. works great. no more clangy annoying metal sounds or vibrations (and it was free since i already have it).