Hello, i bough my first set of rats today, they are both white and black females called molly and milly. I know that rats are not supposed to have saw dust in their cage, so mine are currently on wooden pelleted cat litter, which the lady in the shop i bought them from said was the best thing. The only problem is its really noisy where they are running all over it. Is there something safe that i could put over the top of the cat litter to make it quiter? Thanks for any help.