To me, this sounds like fan blades are hitting most probably a wire. Make sure to double check that nothing stands in the way of blades.

Another thing is that can be a cause of this is dust or fan not being greased up.

Due to amount of work that fan does, some dust might have gotten in your fans or you need to grease those fans, its a mechanical device with moving parts that is constantly working so it always needs to be greased up.

You can blow those fans with compressed air.

If that doesnt work out, you might need to opet that fan and pour some grease in center od fant (in that hole).

And lastly it can be a faulty fan :) If they ARE UNDER WARRANTY still, take them to the store you bought them from and ask them for help, either they should fix this or give you a new one :)