So after a launch one day this week, this noise started up. It only happens on decel, nothing on acceleration. I can't tell which area it's coming from. I have changed the rear diff and tcase fluids one at a time and the noise remains. Waiting on Trans fluid to get here to try that. Here is a video of the noise.
Making about 750hp through this-
Fully built 2.4l
TRE Trans
TRE/buschur tcase
Stock rear diff

 

Taken from Jack's Transmission webpage.

"It is very important to know your Transfer Case is full! EVOs are especially sensitive to this and if it is not completely full you will burn up the expensive ring and pinion gears in the unit! If you fill the unit via the factory fill hole in the T-Case, it can fool you into thinking it’s full when it is in fact not. The small oil sump in the T-Case will make the fluid back up in the unit while you are filling it and come out of the fill hole to make it look like the unit is full! Do not believe this as it can cost you a T-Case!!

If you do not follow these instructions perfectly and run the T-Case low or with any fluid not recommended above you can kiss your t-case good-bye! 99% of the time when a customer has a whining T-Case it is due to either low fluid or the wrong fluid!

Proper way to fill T-Case when unit is in the car:

A) Remove the DRAIN plug.

B) Take a hand pump and insert the hose into the drain hole. Tape, clean rag, whatever can be used to try to seal the area the best you can. Kevin M. found that a 3/8" pipe thread fitting you can find at any hardware store will fit and seal. Get one of those that will fit your pump hose with a barb fitting and you just found a great way to fill your T-Case safely without making a huge mess! Pic of it at the bottom of this page. Crude, but it works great!

C) Pump a full quart of fluid through the DRAIN plug!

D) Quickly remove the hose and install the drain plug.

E) Drive the car around the block.

F) Park the car and remove the FILL plug from the T-Case so excess fluid can drain to achieve the proper level in the unit.

G) Install fill plug and now you are 100% sure it is full and the ring and pinion will be happy.

Filling through the factory fill hole:

This is very dangerous and we DO NOT recommend it as it will fool you into thinking it’s full!

Fill the unit keeping track of the amount of fluid going into it. If it backs up and looks full, install the fill plug and wait 10 minutes. Pull the fill plug back out a repeat until you are sure you have almost one full quart of oil in the T-Case!"

 

After talking with Buschur, it is most likely tcase beginning to go, for anyone with a similar question in the future.

 

http://www.evomoto.com/tech_info.php?tPath=2&tech_id=25

This article says fill from the fill plug.

 

so what was the issue? was it the t-case or the transmission? I have a 2005 mr with the 6 speed with the same issue but i was told that most likely it could be the transmission.

 

Back before I owned an Evo (purchased car in January 2004), in the mall one day I stopped by an electronic game shop. They had a rally simulation. Loaded on my computer, I wanted to drive the Evo. The developers had recorded actual sound from various cars. The Evo in the game had this terrible coast down noise. When I got my car I started hearing the same noise. 100K later it still does it and it still moves under its own power.

This is just my theory but I think the first Evos to come to the USA were built loose in the gearbox department. This is probably good for a car that gets driven on the track and gets hot. However, I think it also was the cause of the p0300 problem. Notice later cars have interchangeable parts with the first USA cars but they don't suffer this problem. I think what changed was that they built the gearboxes to tighter tolerances. This probably also explains the incidence of transfer case failures.