I have been woken up in the middle of the night about 6 times this year, because my HOB will start making a very loud buzzing sound. It's almost like the noise it makes after it's been turned off and then back on, when it's trying to filter water but the water isn't flowing but I think it's coming from the propeller or whatever it is that does the initial filtration. I assume it's something getting caught there? And the only way I've been able to stop it before is by unplugging and then plugging it back in, but now even that's not working. I'm hoping it will go away with time, I don't really know what the real problem is, do any of you know what it might be or have similar experiences?