Just noticed loud pop through speakers when

[Hershey]

starting the car . The stereo has been off when it occurs . What's with that ?

[TPA5]

Sometimes a voltage surge can cause a popping like that; sort of like plugging a guitar into an amp when it is on. Not sure what would cause it in a Yaris, check your fuses perhaps? Maybe check the wiring as well to your speakers to ensure a short isn't occurring.