Turns out the hissing noise is a norm for Lancer EXs. Who knew? Harry_aq, Bracket, can you guys shed any light on this?
http://www.lancermotoringclub.com/showthread.php?12819-Hissing-sound-from-under-carriage
http://www.lancermotoringclub.com/showthread.php?45505-Hiss-sound-from-undercarriage
http://www.lancermotoringclub.com/showthread.php?42475-Hissing-Sound-from-undercarriage
http://www.lancermotoringclub.com/showthread.php?21510-buzz-buzz-sound-at-the-rear
http://projectlancer.net/forums/topic/20913-fuel-pump-making-a-hissing-noise/
Jor-el's post in Strange Hissing Sound? was marked as the answer