Here is what you are looking for:
3M™ Underseal™ Rubberized Undercoating, 08883
Take off your rear wheel, mask, prep/clean, and spray this coating on as it will help in what you want to accomplish; Line-X, et al, dries too hard. The 3M product is flexible and will not fall off the inner wheel liner. You will be establishing a different layer to help dissipate/absorb the sound wave as it travels through both the rubberized coating and the wheel liner; plus this should actually adhere to the liner.