Engine is Off, But Noise Still There

I have a 2015 Corolla CE model. After driving, i turn off the ignition and remove keys from the ignition switch, 10 seconds later there's another click that shut the car down completely. But lately, after remove the keys from the ignition , that second click does not happen and there's a low buzzing noise coming from the bottom of the car. I checked under the car and it appears to be coming from a black rectangle box below the rear driver side. The noise only goes away when I reinsert the car keys and turn it to accesory mode or turn on the car and then turn it off again.
Please help.

I've had problems with mysterious noises under my car on startup and turn off.......wha I've done is recorded the sound just incase i needed to make a claim.....but than over time i just learned how to recreate the noise and i took it in to toyota to show them....and they heard the noise and apparently took apart my car to figure it out....long story short they gave me a long bullshit story about how its normal....so i still record the noise when i get a chance...

thanx for your help

Did they give you a print out saying detailing your claim and their response? Make sure you hold onto that with the audio.

thanks.They noted the problem on the invoice. yeah, they gave me the same excuse you got, that's it's normal for a new car.