If the noise is coming from the SQ:
Could be a fan bearing that is bad that quiets down after it gets going.
If the noise is audio coming from the speakers:
The speaker system should always be the last thing to be powered up, and the first thing to be powered down. It could be anything in the signal chain, but when powering up, always move in the direction of the signal chain from the source to the speakers / in-ears, and when powering down do the reverse.