Ever since I put on my 3" exhaust I've noticed a whistling noise everytime I let off the gas/shift. It only does it when I'm not into much boost, maybe only a few psi.Anything above that and I get the usual whoosh noise from the 1g bov (recirculated).
The noise almost sounds like it's coming out the back of the exhaust, but it's hard to tell exactly.

mine does it too. when I was venting it went away, but since I got another recirc. pipe and put it on the sound is back.
for me I could hear it comming from around the intake filter....

it could just be the bov partially opening

Mine does that too. Under a couple pounds of boost it makes a really blowy kind of sound. Then if you step on it, it stops and the normal BOV noise happens.

its most likly compressor surge coming from the turbo.. (sounds like a whisstle) because your blow off valve is not releasing the excess boost when u let off the gas at such low throttle.

It usually happens when u have aftermarket bovs for high psi applications. The bovs dont open at such low boost

thats my guess..

This isn't likely the case with a STOCK unmodified 1g BOV they leak pretty early into vacuum like 16-17inhg... somewhere along those lines.

You may compressor surge a lil with a very crushed BOV in this situation. One that was crushed not to open until like 20-22 inhg...

Nope, not the turbo. It's the BOV. All the stock ones do it to some degree, esp. when vented. At low airflows it acts like a whistle, as the air hits the sharp corner from the inside of the BOV body to the outlet tube. The sharp edges surrounding the valve and seat also contribute to this.

I've hacked up a few BOVs to increase their flow, and during the process all the sharp corners went missing. The high pitch whistle at low airflow went away completely. It also moved a LOT more air when fully open too.