i have a 04 wrx with a vf39 turbo and just last night the turbo started to sound a little weird, now it sounds like it is building up boost but isnt letting it out and making like a hissing noise or sounds it like theres a leak or something, it still makes a little boost but you cant shift over like 4k
Honestly, it sounds like a leak issue, on my DSM that happen, but it was two things, one my ecu was losen power from a bad ground so I had to kick it to make it drive again, or you have a boost leak issue some where.
No Em? I would check all the hoses like mentioned, and start with the Turbo inlet an work you way to the throttle body. Could be a uppipe leak as well. Look for carbon building up around the Exhaust connections.
it may also be the waste gate actuator causing the boost problem. the hissing sounds like a boost leak but i cant tell for sure without hearing it myself. also, your bov may just be opening prematurely. good luck