I've picked up a lot of good used HOB's cheap 'cuz of people getting fed up with noise problems and the majority of the issues seem to all relate to the impeller, which makes sense since it is the moving part. Penguins, AC's, Whispers, and even Emperors will all run near silently, water discharge splash notwithstanding, with or without lids.
Deposits in the impeller well, on the impeller magnet or shaft, all can throw the high revving device out of whack. Normally I'll start by soaking the impeller in white vinegar and fill the impeller well with the same. Sometimes there can be scale or mineral deposits that you may not even be able to see. Let it soak over night and then give it a good brushing. Some of us older more sight challenged need to use a magnifier but checking the impeller for twisted up strands of filter floss and plant material can also sometimes dispatch the balance phantoms.
Check the impeller shaft for straightness and scoring. Also check the magnet end on for roundness and concentricity. I've reconditioned more than a few that had rub spots worn along one side from being installed wrong or just being out of shape from the factory. With some of the Penguin impellers I've actually gotten away with sanding the plastic around the magnet to get it in round again. I didn't expect it to last but I've been running one for a year like that.
Wrongly installed Empire impellers will often mushroom the bushings on the ends of the impeller shaft. Apparently these are made to fit quite snug and this contributes to the problem. I've dressed these up with a little Xacto knife whittlin' and emory board action successfully. When the intake is assembled the impeller will not be able to fully seat with the misshapen bushings. This binds the impeller up and causes...noise, or no function at all. This is a problem that I have seen on every "noisy" Emperor that I've worked on.
CWO4Gunner's got a great post in this forum for AC shaft replacement if you find a bent or disfigured one. He's also got some interesting mods for the AC's. Maybe he'll give us the secret to the stare.
Lastly, if everything's clean and properly shaped try a tiny dab of silicone grease. Dive shops usually carry it much cheaper than aquaria suppliers, if they have it at all. It is inert and will not poison your fish or inverts, just remember that you're looking for silicone grease not caulk or adhesive from Home Depot. A little goes a long way and a tiny container will last a long time. It's also very handy for lubing o-rings and o-ring sealed plastic lids on pressure vessels.
I should try that someday. 
