First, Vantecs aren't as quiet as they claim. Second, adapters are usually bad since they cause more pressure to build up behind the fan = more noise. Third, fan noise includes also the vibration caused by the fans, if you have several fans the vibrations are compounded and so is the resulting noise. Fourth, GPU fans are small, and small fans = high dbas per cfm.
To quiet your system ---
First, with your comp off and cooled, unplug all your fans, even the ATI and the CPU fan. That way, only your PSU fans are plugged in. Turn on the comp (for a little while, don't want the cpu to burn) and see if it's loud. If it is, either clean the PSU fan inside-out or replace it all together. Both requires you to break the warrantee for the PSU. If its not loud, plug the cpu fan and see if you can hear it. Keep doing this till you find which fans are loud and need to be replaced.
Chances are, the loudest fan is the one on the ATI. You can get a fanless heatsink for the GPU from Zalman. Or you can just get the Crystal Orb and do the 7 volt mod on it. For your Vantecs, you can do the 7 V mod too (although that will hurt your cfms), add rubber grommets, add noise padding, the list goes on. Search the forum for more answers.