Why does battery dies when truck sits for a couple of days? on 2008 Hummer H2

When I hook up charger there is a faint buzzing sound coming from a small canister near the fuse box. It will buzz for a few seconds then stops.

Asked by for the 2008 Hummer H2
have the battery tested (high rate discharge test and or hydrometer test)an inductive amp meter would be needed to see if there is a parasitic draw on the battery (something staying live when the ignition key is off).
1 more answer
Since all batteries create power by a chemical reaction, we say that the chemicals have exhausted themselves in the reactions. This would mean that a dead battery is exhausted.