Tommee Tippee Sippy Cup Making Loud Air Noise and Cap Loosening

We bought a Tommee Tippee sippy cup two weeks ago, and while my twin daughters have no issue drinking from it, there’s a loud air noise every time they take a sip. It’s not a minor sound—it’s loud enough to wake one twin when the other drinks at night. It seems like air is coming from somewhere, but I’ve never encountered this with any other sippy cup. Given that both cups have the same issue, I don’t think it’s a defect—there’s definitely something wrong with the design. On top of that, the cap keeps coming off in their hands, which is frustrating. We paid quite a bit for these cups, and I’m disappointed we chose this brand. I wish we had gone with a more reliable option.