AquaClear mini and 200 problems...

My mini and 150 are both silent. My 300 is very noisy with vibration. I have to keep the lid on crooked or else it rattles and drives me nuts. I don't know why they do this, and like you, I'd really like to know why some are quiet and some are noisy.

Sometimes if there's debris in the motor (grit, for instance) or the impeller shaft isn't perfectly straight, it will rattle. I have the same problem, after a few years one of my ACs suddenly started to make grinding noises. I think you can get replacement parts, but I'd try contacting the manufacturer first to see if there's anything you can do.

My AC 200 is so quiet I have to look sometimes to see if its running, I keep the water an inch or 2 lower than the top of the aquarium so I can hear the sound of the water, when it starts to trickle I can hear it and know its time to unclog the intake pipes. Just one night it was making a buzzy noise and I cleaned the pipes and put it back, it was running fine but still buzzing. My room is near the tank so, I was falling asleep listening to the buzz, and then the buzz just stopped. I got up, thinking the thing had stopped running altogether, but it was ok. Its been quiet ever since. I stick a wooden skewer into the impeller everynow and then to clear the junk out. One time it stopped completely (no sound from the motor at all)and I panicked and ran out and bought another, only to come home and find it started working and has been fine since.
I like the AC 200 for its simplicity and it does a good job of filtering, but next time I might go with Fluval. This one freaks me out too much.

My 200 does the same thing. Usually I find that it does this because it's clogged in some way. So here are the things I normally do which seem to work:

1. Clean the pre-filter sponge. If the filter is having a hard time intaking water, it will buzz.
2. Take it apart and rinse out the impeller and it's little compartment really well. If there's sand in it, it will buzz.

Sometimes it will buzz right when I start it up, and then it usually stops within 30 seconds or so. If it doesn't, I play with the level of the water flow, until it stops.

And I agree, it's a nice little filter, but I'm going with something else next time. With a planted tank and sand, this filter has been nothing but a hassle. It actually stopped completely just about 30 minutes ago. I had to take it all apart and clean it out, and now it's fine again. Looks like I'm going to have to do that at least every 3 weeks or it will just stop suddenly.

Good advice 12, all the more reason to have a back up in case your out when it decides to stop.