I bought this to use with a 21oz Hydro Flask (with standard mouth), so I wouldn’t have to fuss with unscrewing the bottle when using it at the gym. I am loving both the bottle and the Coldest Water sports cap. I use the straw (included with the cap) and have not experienced any whistle (or other) sounds. The flip top is easy to open with one hand and the loop at the top makes it comfortable to carry the bottle. Great product!
Review by on January 29, 2016