I have bought 3 of these, and one of them keeps making a sound as if something is knocking inside. I opened it and cleaned it, maybe it had something it shouldn't have, but it still does the same. I'll see how it goes, I hope it doesn't stop. But if out of 3 one turns out like this, you can't exactly call it a perfect purchase. If it weren't for Skroutz's warranty of proper product operation, I wouldn't feel secure with my purchase