A fan that is very close to the vent holes of the side panel creates air turbulences. These turbulences can be very noisy. The noise level depends on fan size/count, fan speed, fan blade design and fan distance to the panel.
The easiest way to reduce this issue is using micromesh dust filters. DEMCIFilter offers two custom sets for the A4-SFX. If you like to test this solution temporary, cut the tights of your girlfriend and span it over the fan.
Alternative solutions are, installing thinner fans or reducing the fan speed.