That's normal in any power amplifier unless you have somehow guaranteed DC-balance during startup...
If it bothers you, I suggest you look at adding an output relay to provide DC protection to the speaker - you can find details on Rod Elliott's excellent Elliott Sound Products website. This will prevent startup and shutdown 'thump' affecting your speakers.
If it bothers you, I suggest you look at adding an output relay to provide DC protection to the speaker - you can find details on Rod Elliott's excellent Elliott Sound Products website. This will prevent startup and shutdown 'thump' affecting your speakers.