It is not normal.
It happens because your receiver does not properly mute its outputs while it is powering up. Until the receiver has properly powered up (initialized and stabilized itself) there are noise transients in the audio circuits -- and this is why it is supposed to mute itself. They will vary rather randomly each time you power up, so that may be why you don't hear it all the time. Or it could be that the power-up muting is happening properly sometimes. Or it could be that you will only hear this if the receiver happens to have a live audio source selected when it powers up.
This could be a fault in the receiver or it could simply be poor design in the receiver.