Occasionally when the flip flops are brand new, they can squeak if some sweat develops underneath the foot. The squeaking will typically go away once the flip flops are worn in, and some of the initial sheen wears off.
In the instance, the flip flops are squeaking with normal walking (they are not wet), there are two most common causes:
Sweat may be developing between the flip flop and the foot, causing the squeaking noise. The flip flops have come into contact with something that is causing them to squeak (normally dirt, microscopic dust or a chemical agent such as shampoo or conditioner).
If your flip flops are squeaking and it's not because they are wet, then we suggest:
Wash them in warm water and give them a vigorous scrub with a scrubbing brush or green scourer then thoroughly drying them with a towel.
We often find that when this happens, the flip flops have somehow come in contact with something (we don't know exactly what) and washing and drying them often fixes this!
If the flip flops continue to squeak during normal walking, after they have been well worn in and after you have vigorously washed them, normally 'roughing up' the surface of the flip flop with some coarse sandpaper fixes this.
The flip flops will usually squeak when wet such as after swimming regardless of whether they are brand new or well and truly worn in.
However, once the flip flops are worn in, under normal circumstances, they should not squeak during normal walking (unless they get wet).