If the car is awake a lot of stuff is turned on causing a
drain on the battery . One of the things that is on is the amp
if you lock the car it should turn off within a minute.
Locking the car causes it to go into sleep mode faster also do not
leave the FOB close to the car as it can detect it and this will cause
the car to draw more current.
I would get that noise fixed or looked at though because it shouldn't
do that unless you have the volume almost floored on the Stereo .