who thinks they know what this wierd sound is?

[blueb8llz]

this is on a 3905 emaxx. it mades a wierd noise everytime i accelerate. the noise sounds like a old door or floor creaking or like something stretching, or dead leaves being stepped on. odd thing is, it only makes the sound:
- when you go forward. reverse doesnt make sound
- when i lift up the truck and accelerate, no sound
- when i lift up my truck and compress my springs to mimic as though the truck is on the ground, no sound
- when i roll the truck forward with my hands when its on the ground, it doesnt make that noise either.
you only hear the sound in the beginning of the acceleration, as soon as the truck goes , the sound stops
its not a grinding noise so i doubt its the gears from the diffs or gearbox.
and the sound is coming from the rear of the truck positively.

heres a a sound bit no video sorry:
http://home.earthlink.net/~webhost/traxxas/emaxx.mp3
picture what i was doing with the emaxx...mic stand holding a nice 3/4" diaphragm condenser about 1' off the floor, down slightly from horizontal. emaxx backs up to the mic and i jerk the trigger and repeat