Banks don't care and don't want to know what is in your safe deposit box as long as it's not a danger (chemicals, explosives, etc.) to their staff or equipment. Mainly because if it's illegal and they know about it, they have to report it, and that's bad PR. The down side is that, even at the cost of the silencer, the ridiculous SD rental rates would probably be more than the silencer in no time at all.
When the rent becomes delinquent and the locks get drilled, you would be amazed what is found in them; cash, jewels, guns, nude pictures of the wife/girl friend. You name it and it's in there.