Our Tri-Tech Sports lid may make a whistling noise as the stainless steel isn't pliable or squeezable. Our Tri-Tech lid has been designed so air can get back into the bottle as you drink. The noise you hear is the air moving through the valve. Older Tri-Tech lids do make a whistling noise when used. Our recently designed Tri-Tech lid is significantly quieter, however it may still make a little bit of noise as the air passes through.