That's a very familiar sound. I battled that specific boost leak quite a few times especially after Stage 2 boost levels.
My money and full confidence is on the intercooler outlet to throttle body silicone connector. For some reason Subaru has decided to use a harder silicone hose with little softer silicone "skirts" (I don't know what else to call them) on the ends where it clamps down. The softer silicone simply slips from under the clamping force causing a leak. This is why the leak sounds like pulling a balloon neck apart and letting some air out.
The only thing that solved this for me was eventually removing the softer silicone skirts and just clamping down the harder silicone hose... but my setup is quite different from stock now (I just happened to get lucky to use the same coupler for my WAIC unit)... I also run around 20 psi of boost.
Anyway:
1). remove the two bolts that connect the intercooler to the turbo on the passenger side
2). Remove the bolts on the metal bracket that hold the intercooler up on the driver side
3). Using a long screw driver, or better yet a 1/4" ratchet with a long extension and I believe an 8mm socket to uncrew the silicone hose from the intercooler outlet and pop that bad boy off.
4). Completely remove the hose (keep track of which way it's facing and which end is which so you can put it back how you found it
5). Wipe or wash the hose inside and out, it will have some oil in it.
6). Wipe intercooler outlet AND throttle body with rubbing alcohol to degrease it from any oil.
7). Wipe both ends of the hose down with rubbing alcohol to degrease the little silicone skirts (or better yet, rip them off.... and throw them away, or get an aftermarket one).
If you want a peace of mind, ORileys has similar but much beefier hose clamps (they are located out back, you have to ask an employee to get them for you). They work much better.
Reassemble and tighten the hose clamps as much as you can without breaking them.
Here's an aftermarket part:
http://www.ebay.com/itm/MISHIMOTO-T...ash=item41958ac676:g:1~8AAOSw14xWHtR6&vxp=mtr
By the way, here's a pic of what the wonderful silicone "skirts" do sometimes... I hate this part.