I recently purchased a fan controller for my case fans to reduce the noise when I don't need a lot of cooling power. However, it's actually made it worse. When I decrease the fan speed with the controller, it does reduce the sound of the airflow BUT it increases the sound of the fans themselves. The motors of the fans make a slight growling noise when not running at stock speeds that that actually creates more noise than running the fans at full speed. Anyone have any recommendations?
Check the fan mounts and make sure that the screws are snug (no need to over tighten).
Another thing that comes to mind is resonance or natural frequency of some of the components of the case; particularly the sides. This can be addressed by attaching some dead weights to the sides on the insides.
I had not even considered it might be the mounts or the case rattling against the mounts as you suggested. So i took the fan off the case and tried the controller just holding the fan in my hand and it still made the noise. After doing some reading around, I see that some fans don't actually under-volt well. Too bad since they are actually pretty good fans that move a decent amount of air. Just so much air that it is a little annoying when my PC is just idling or I'm just surfing the web. Supposedly Noctua's under-volt well so I may pick up a couple of those (dispite their ugliness).