What is this thing?

[ChevyHighPerformance]

Mine was rattling too. I unbolted the canister and put rubber pads where the metal canister and the bracket touch. I also bent the rectangular spring tab on the bottom of the bracket so that it loads the canister tighter when bolted down. I wire tied the main black hose to the bracket to keep it from wiggling. There is a slight rattling sound coming from inside the connector/valve that I'm not sure you can do anything with.