The engine is cooling down. (or at least that's one reason for it)
the a/c compressor cluth is bad. should make a grinding noise when not on and goes away when it is on.
because the engine is still processing the oil for about 20 seconds after you turn the car off
The automatic electric antenna is retracting or trying to. It should be silent after it has fully gone down.
Possible vacuum leak
Sounds like a loose alternater belt.
because of you
Try taking the power lead from you radio and run a wire directly to your battery. This will bypass anything that will cause noise on your radio. Just remember to turn it off when you get out of the car!
Put Key in ignition and turn it a bit.
Maybe the fan cooling your car off.
turn on and off the device and when you do that listen or feel the relay switch it should make a noise you can feel it too, if you can't hear or feel anything that means it is broken.
The 2001 Nissan Altima GXE might make a clicking noise and then turn off because of a bad solenoid or bad starter. This sound could also be due to a weak battery.