fortysixandtwo

As Techno Duck said, the springs need to be set properly. Most will work right out of the box, but if the BOV or diverter is making a fluctuating noise, this is the indication of a too stiff of a spring. Although its relieving pressure, there can still be a compressor surge condition.
The type of operation you want out of the valve is a single, nonfluctuating release of pressure. i.e.: psst, instead of psstststtt

There are a ton of different model BOV valves and many of them can be configured as a diverter valve. They're all going to have a different sound.
I have a Turbonetics Godzilla BOV, it makes more of a sigh than a whistle or chirp noise, when shifting under heavy throttle. Shifting while at low boost is still noticeable, but I don't get any turning heads.

Techno Duck

One thing I noticed is with the Evo motorsport bypass valve I get compressor surge (flutter dump) when at full boost only when I lift. This is with the stock spring. It's weird because it was a non issue with the 26/6 but after I went to 60-1 it started, I guess it's just the extra CFM it's pushing. NZ951 reported the same thing when he went with the 60-1 and same as my friend whom I bought the turbo from. I plan to buy the Forge valve so i can play with the spring pressures, or pick up a Greddy Type S if I can find one.