I guess I got real lucky with the Aquaclear 70's because these HOB's have none of the issues/problems that Mikaila wrote about.
The AC series has given me nothing but problems and not just with noise. I have owned an Aquaclear 70, and an Aquaclear 110 (this being an emergency purchase when a filter died and I had to replace before going out of town).
The Aquaclear 70's first motor had a good long run. It fit properly on a 75 gallon tank but when the move was made to a 150 I had to fight it every time it was unplugged for a water change/maintenance. It developed major noise issues, began running hot, and eventually died. It didn't help that it didn't seem to want to stay upright when placed on the 150. Interestingly the AC 110 also doesn't seem to want to fit well/stay up right on the 150.
It went into storage, and later on I bought a brand new Aquaclear 110. Within a few days after running I got a call while out of state that it was making a lot of noise. As time went on the noise became so irritating that it was driving people out of the living room. I eventually replaced it with a canister.
The AC 70 got brought back to life with a slightly used motor I found. It too went the same way as the last. Only it started out quiet and far sooner began to have major issues with noise, and over heating (this was on an 80 that it for some reason has issues fititng as well). It has yet to burn out but I am not running the risk. It is now replaced with a Sponge Filter. I know others who have similar issues with major noise complaints and I know the company that makes them was on back-order not to long ago as multiple people were reporting issues with the motors and sending them back to be replaced creating a wait.
I didn't appreciate spending 100 bucks on what was supposed to be a good filter only to be told I had to send the whole unit in to have the noise looked into and then leaving my big tank short changed. Plus I was informed of the back log. A noisy filter was better than none at all.
Some people speculate that they are currently making their stuff far cheaper and it has led to a dip in quality hence the issues people are having.