I hear a hissing noise on some of the products. Is that a defect?

No, what you are hearing is “pink noise” (sometimes described as a swishing sound). It is a combination of all the frequencies the human ear can perceive, with higher and lower frequencies adjusted to be equal in volume. Pink noise enhances the perception of the Hemi-Sync® sound patterns, and therefore plays a valuable role on certain Hemi-Sync® products.