There are high performance fans out there that stay quiet at full speed. However they are pricey. At a certain point the amount of air moved will be noisier than the fan itself- There's nothing you can do about that, but slow them down and in turn reduce the amount of air.
TLDR; performance cooling at low noise requires some sort of fan control - be it a fan controller, a software solution (like speedfan), via motherboard or plain 5V or 7V adapter cables - and high quality fans.
For example, some good ones are Cougar Vortex or Enermax Fans (like T.B.Silence Manual).
Keeping your side panel of is a crap "solution".