It's your bypass valve operating.
@the duke
also, i wanted to ask about the fac that it only vents sometimes. is this normal as well?
It's based on a number of parameters the ECU is receiving feedback for. Pressure, temperature, throttle position, load, spark, speed, etc. The ECU is looking for the best drivability and this includes when/how to vent the intake charge to make the car more responsive. We do not have traditional mechanical bypass valves with a simple spring. It's a solenoid that the ECU controls, and adjust as necessary quite quickly.
In that case you should not have any flutter at all like I explained, only "pshh" and "whooshing" sounds depending on if you're in boost or not and the lenght of time you keep the throttle closed. It's only a spacer and it can only be bolted one way so there's nothing that can go wrong on that part. If it was leaking because of bad machining or incorrect tolerances, you'd hear the leak constanly under boost, it would not affect the behavior of the BPV.
The BPV is either fully opened or fully closed, no in between and it will open every time the throttle is shut off, no matter if you're WOT/full boost or low rpm/under vacuum/cruising. Check that the electrical connector is fully seated on the solenoid.
Can you take a clip of the sound? I'm really curious to hear it.
You could also ask your tuner if he did something on the tune that could affect the behavior of the solenoid. I don't know if they do that kind of thing.
Under specific load and unload, the OEM bypass valve will flutter a bit. I was able to match the condition to get it to flutter even when stock. The BPV is not either fully open or fully closed. It's an ECU controlled solenoid and it can oscillate based on demand feedback into the engine. That's the whole purpose of it being electronically controlled, there's much finer resolution the ECU will adjust for. At partial throttle and lower-pressures, the car will ride the line between needing to vent pressure and wanting to keep the bypass valve closed for response. It's adjust this every time you let off the throttle.
Old spring-type bypass valves (or BOVs) are purely pressure based, but even those can oscillate due to pressure spikes as the spring stretches, the force pressure is vented, and thus closed, causing the pressure to build, repeat, etc.
There are two main groups of turbo noises.
The “PSHHHHHH” sound is the excess boost being vented to atmosphere by the BOV you just had installed. It is 100% harmless. With the stock setup, it was fed back into the intake and inaudible. To answer your question, you won’t hear it all the time …. only when there’s enough boost built to create the noise, and you come off the throttle in such a way to create intake vacuum to activate the BOV (which can be mechanical or ECU controlled, depending on type).
The second set of turbo noises ….. this is a higher pitched sound. Sometimes it’s described as a “STU-TU-TU-TU” noise. Other times, it can be just a quick, high pitched screech, depending on boost level, how quick you’ve come off throttle, and a host of other factors like intake type, exhaust headers, etc. This is the sound of the turbines “surging” because you are slowing them down suddenly from high RPM while still under boost pressure, ie before the BOV can do the job described above. Yea, that’s compressor surge. While it technically is causing additional wear, it’s not the sentence of death it was to turbos 30 years ago, and the wear issue is often exaggerated by internet lore. Many folks go out of their way to get these sounds and, conversely, they can be nearly impossible to eliminate if you have an open intake, open exhaust, etc.
Bottom line, if you don’t like the noises, I get it. But, don’t dislike them because of anxiety.
Real compressor surge is due to the over-pressurization of the intake system while under constant load. When this happens, the engine can't consume the air enough and the boost builds up in the intake, regardless of the throttle-plate being open. This starts to cause a rear-flowing pressure wave/bubble that then propagates back into the turbocharger compressor, which is spinning at some 80K rpms as it's connected to the turbine whizzing at the same speed. So the engine is making power and positive pressure (boost), meaning the turbocharger has a more than adequate amount of gases moving through the engine to create enough energy to spin the turbine wheel, thus spinning the compressor wheel. While also having higher pressure spinning against the compressor wheel. This is what destroys turbochargers, as you get unbalanced torques, vibrations, warping/bending due to the mismatched and oscillating pressures. You can literally torque/twist components spinning at 80K. Carnage ensues.
The noise the byapss valve makes means nothing. It's a pop-off valve, nothing more. The design of the port, spring pressure (If a fully mechanical system), can influence the sound, but it only has to vent pressure over a certain PSI. Without a bypass valve the intake charge will simply be consumed by the idle air circuit eventually, but it's certainly better to have the pressure vented/bleed off quicker.
Unrelated, but if you're interested look into the Prodrive Rocket from their glory Subaru WRC days in the mid 2000s. They even patented the entire design so it's in the public space. It's a giant middle-finger to bypass valves, and it's one of the most ingenuous ideas I've seen in Rally in decades.