seen this noise a lot on AWD dsms
Its all about drivetrain harmonics.  mitsubishi did not enginier in a driveshaft damper like other companies.  they opted for light weight and small.  because of this it reverberated the noise out the driveshaft.  I have had my awd on a hoist.. with it in gear making the noise and it sounded like it was coming from the driveshaft... listen closer and you can hear it coming from inside the tranny where the center diff/or front diff is.  Also the noise was not noticable at all on acceleration... just on decelleration.. i have delt with bad tranny bearings before... several times in fact... and when that happens you can hear noises on acceleration.. thats when you know the trans is screwed.
  The noise stems from the backlash in the gears in either the center diff.. i dont think its the front diff as i have never noticed this noise in any FWD turbo DSM's... alwaus AWD ones.   It is accented with lighter flywheels and stiffer clutch disk springs and balance shaft elimination.
Good luck trying to make it stop... i had this noise on my 1ga awd and i replaced EVERYTHING (t-case, driveshaft, rear end, tranny and engine....and it still did it.  when i changed the tranny the noised changed a bit.... but it was still there.
A tranny rebuild still wont help unless you buy a brand new center diff.  It could even be from the front differential.  As long as your fuilds are not filled with metal filings your fine.  oh and i also found that going to soft squishy winter tires helped dampen the noise a bit... go back to low pro summer performance tires and the noise got louder...
Turn your radio up louder 
