This issue has been here since the launch of the game and I hope it can be fixed at some point.
For those, who do not know, when the throttle is released with a saturation of more than 1, the blow off valve will not sound. Obviously, this only applies to turbo charged cars and not supercharged or naturally aspirated cars. As far as I know, this applies to all platforms as well.
To help the devs along, here is some pseudo code that they can use.
TurboBoost is Integer
Throttle is Integer
Blowoff is Boolean
If TurboBoost = 100% Then
Blowoff = True
If Throttle = 0 Then
If Blowoff = True Then
Play Sound "Blowoff valve" (note: can be randomised from another algorithm)
Blowoff = False
This isn't a perfect algorithm, but it should help. I hope that this code helps the bug get fixed sooner.
Thanks for your time.