My Nuk 5 hisses when I suckle it. Is this common or do I have a defective one?
Normal. Mine is making that noise right now
The sound is just air being expelled from the latex bag when you suck on it. All three Nuk 5's and all I-dunno-how-many Nuk 3's I've had have made that sound to some extent.
Mine does it as well...I'm pretty sure it's normal!
(especially since like Tygon said, the smaller ones do it to!)
Maybe you have a small microscopic snake inside it? Mine doesn't hiss.
Considering they're not solid and they're not a sealed system, it's not unusual to get some sound when you compress the air out of it. I guess if you maintain or completely avoid pressure on it at all times, it'll keep quiet. Doesn't bother me in the least.
yeah this is normal, it only happens when the air is pushed out of the nipple itself though. I only suck on it lightly if I do at all and I never get the hissing sound.
But you shouldn't be concerned about it at all, perfectly normal.
It's normal, if you've sanitized them with a gentle 5 minute boiling they'll gurgle for a few days.