When I turn the key on my corolla, there is the sound of decompression of gas

would it be the fuel pump coming on in the tank for 2 seconds,and then turning off.if thats what you hear,its normal,unless its real loud.pump could be blocked and working to hard,or failing pump.

Brake booster vacuum... when you turn key on you are pushing brake as a natural reaction... NOTHING to make that noise simply by turning key on!.. Not even the fuel pump, got to have cranking (distributor rotation) input for that to turn on.