If your foot massager does not consistently generate the louder sound you are referring to, the most likely answer is that you are applying too much pressure against it. When using your personal home foot massager, be sure you just relax and allow it to do the work.
Do not apply pressure against it with your feet or legs. You will also find it to be much more soothing and therapeutic when used it in this manner. This is, after all, how it is designed to operate. By applying pressure to the unit, you may inadvertently cause damage to the unit as well.