My Kegel balls make a rumbling sound when I move around, is this normal?

Question

Do these Smart Balls 'rumble' or are they just dead weights?  Is it normal for weighted Kegel balls to rumble?

Pelvic Floor Answers;

Smart Balls  have a heavy weighted ball inside a sealed outer medical grade silicone ball and the inner ball moves around with movement, this is part of the toning process and is perfectly normal.The rumbling sensation is caused by the movement but does not produce noticeable sound.

It is possible with some brands of Kegel Balls that are not made with quality materials- where the outer ball is a hard material such as plastic, that the inner ball is making a sound when it connects with the outer ball. 

This is not an issue with Smart Balls