Most likely it's caused by vibration. SATA drives are very sensitive to vibration. In some case, vibration may cause 90% performance degrade, and even drive offline. In a 1U chassis, high speed fans may generate chassis vibration. That's why enterprise SATA HDDs are always recommended, which have better tolerance of vibration. I'd highly recommend you chose HDDs from the Tested hardware list.
You are absolutely right.
After some extensive testing i found that the 2 fans was generating the problem and that it was not an electrical problem.
Logged in here to share my findings as your reply was not here last time i checked the thread.
The problem got so bad that when i did testing with the fans on and off i took one fan off while testing and the raid dropped a drive from the raid and i am currently rebuilding.
My budget does not allow me to change the discs right now so i will have to find some way of dampening the fans so the vibrations from them does not reach the chassis and also do some dampening on the drive bays if at all possible.
Have marked your answer as the right one ... Thanks a bunch for your help...