The holes are only for the crossbars. To fill the support columns, you need to unscrew the cone feet and fill thought the screw hole. The welding of the columns is not tight. Dont use play sand or other fine grain as it will leak through the gaps.
If your goal is to reduce the ringing, I suggest you try isolation expansion foam. It is cheap, easy to apply, light weight, wont leak, and very effective. Just inject the foam into the hole, wait for it to expand, and remove any excess that come out of the hole.
You can also try mixing sand or steel shots with the isolation foam to give the rack a little more weight. Just inject a small amount into the hole, than quickly fill it with sand or steel shots before the foam fully expanded, shake it viciously, and top it off with more foam.
Hope this help and have a great holiday.
If your goal is to reduce the ringing, I suggest you try isolation expansion foam. It is cheap, easy to apply, light weight, wont leak, and very effective. Just inject the foam into the hole, wait for it to expand, and remove any excess that come out of the hole.
You can also try mixing sand or steel shots with the isolation foam to give the rack a little more weight. Just inject a small amount into the hole, than quickly fill it with sand or steel shots before the foam fully expanded, shake it viciously, and top it off with more foam.
Hope this help and have a great holiday.