OK, folks, just found out after a call to Dell Support.
My sound came from the bottom right... which is the cpu area.
Apparently, according to them, this issue sometimes happen to Core 2 Duo, does not happen to pentium, etc. Even after hardware exchange, this prob could still be there. Dunno y
He suggested to try this 3rd party software , RM Clock Utility, and remove a setting called "Enable Popup mode" (under Chipset)..... And it works!!!! No more cpu disturbing noise.
http://cpu.rightmark.org/download.shtml
I not sure what effects will removing this "Enable Popup mode" in RM CLock Utility caused.
I'm ok with this software solution for now.
Note: What might work for me, might not work for u. Try at your own risk. if needs be, just call the experience dell guys. XPS - 24 x 7 phone support.
Cheers!!!