Well, it might not be normal depending on how loud it is. My B&W sub makes a very subtle and quiet noise when it turns off. My PC13-Ultra originally made an extremely loud noise when it shut off (so, like you, I thought that it might be damaging something). So loud that it made the pooch go hide and not come out for a while. I ended up setting it to always be on. SVS recognized a fault in the amp design and has since had the amp supplier redesign the amp. That type of shut-off pop is NOT normal.