Fast Flow Teat is recommended for babies older than 6 months
This has a unique patented skirt system with a one-way air valve at the base of the teat which flexes to alow air into the bottle, replacing the milk at your baby's own sucking rythem and eliminating the vacuum. So your bbay can feed continuously, as at the breast, without having to come up for air.
The whistling sound you hear is your guarantee that air is going into the bottle and not into your baby's tummy. It's the air babies swallow during feeding that can cause discomfort, crying and fussing associated with colic.