Anyone has problem with the 230mm fans in ATCS 840 casing? Why is the second 230mm fan on top spinning at 600+rpm while the first 230mm fan is spinning at 1000+rpm. If I open the side panel, the second fan's speed will increase from 600+rpm to 800rpm. Is it because my air flow too strong?

Hi all pros, I think someone has posted this many times, but this thread got 80+ pages and I don't know where can I find my answer, so I will just give it a shot here.

I have a new ATCS840 Cooler Master casing, there is one 24cm Fan running very slow ( around 600rpm only , another one is running at a normal speed of 1000rpm ), If I remove the side panel, the rpm increased from 600 to 800+, which is very weird ( maybe it is due to air flow ).

I plan to change 2 24cm fans to 3 12cm fans on top. So I would like to have recommendation on which fan is nicer, in terms of air flow and noise, of course I will be getting fan controller, but the market got various type of fan controller, I would like to find those digital panel fan controller. Anyone can give suggestion for fan and fan controller? Cheers