Take off the 2 sponge covers on the outside of the filter box. There is one on the front side, on the bottom right. The other is on the right side of the filter box at the top.
Remove both of these and rinse the sponges thoroughly. Reinstall the sponges with brackets and test.
If that doesn't solve the issue, next try to take the filter pad out and see if it goes back to normal sounding. If it does, then you need to change your filter pad as its clogged.