Why do my Remington wet 2 straight straighteners make this noise?

I bought my Remington "wet 2 straight" hair straighteners yesterday, and as I have very thick hair, have used them 3-4 times already.
When the straighteners are at the correct temperature, they "beep" to tell you they are ready to use. Although today, I turned them on again, and the beep sounds distorted, almost like a battery dying? Yet they are mains only.

I'm worried they are on their way out already?? Can anyone please help?