Ok I was stupid, after some observation it looks like the speaker-look top portion of C side is actually for air intake, that explains why fan needs to run faster when lid is closed.
Hi folks, Just got my 
 
   M15 R6 
  m15 R6 and was satisfied with its noise level under "Balanced" profile in Alienware Command Center. Though it seems after I set everything up and put it aside closed (I used it like a desktop with external monitors and keyboard/mouse), when playing games, the fan runs at full speed and the noise became unacceptable even with noise cancelling headphones. Interesting things happened when I opened the lid, suddenly the fan speed dropped to the level I was expecting under Balanced profile, and ran at full speed if I closes the lid again, regardless of what thermal profile it's at. So it seems the fan behavior is affected by the status of the screen? Not sure if this is by design or not. Hope someone with the knowledge can shed some light  Thanks!
m15 R6, fan speed issue when lid closed