HKS is pretty loud, this is mine:
It's only being revved to about 4,800 RPM, and it's even louder when you're actually driving.
It's not so loud if you're just driving normally, but if you're driving spiritedly, it's loud enough to hear with your windows up. I had a friend in the car who asked about how a turbo felt when pushing it, so I did, and she complained about the "breathing" sound startling her. That was with the windows up.
When I was getting the BoV from ER, they told me that Tial is the more popular choice for its subtlety.