Not sure about that... Velcro is not really that absorbant for vibrations. The material doesn't really stretch. It good to hold thing together (I use it for cable management) but for vibrations reduction...
I have to disagree with Frenchie even though I have never used velcro on the inside of a PC, the idea of having "fuzzy" plastic + thousands of tiny plastic hooks makes me wonder about static.!
Almost anything that vibrates can pass on those vibrations to something that it touches, the sides of a fans and HDD's are the obvious ones inside a PC, just about any way of seperating these items from the case can reduce the vibrational noise (but never the airborne noise). I have used string in the past as well as cable ties and both are better than directly mounting a HDD in a cheap shit case. Therefore I must conclude that velcro will also be better under those circumstances.
Please be careful though as static could be devastatingly bad for a HDD, and also note that there are many ways that will be superior to velcro for reducing/removing vibration borne sounds from a HDD.
I have only even DIY suspended 1 hard drive, that was done with "1mm thick Stretch Magic" and a 2.5" HDD, it was perfectly successful, especially as part of the reason for me doing it was that I had nowhere else to mount the drive apart from a 5.25" bay, which I removed the cover from so it didn't require cooling.