It could also come from one fan blowing air into another fan (CPU fan, for instance). It would make a different noise rather than sound like the fan is spinning faster, though. If you want to see this effect in the extreme, take one of your fans and blow into the fan blades.
Unless it's sitting in open air, the fan will have some resistance that it must overcome from the mesh cutout and fan filter on your case. It may not be much resistance, but it is there. Fans can respond a little bit different depending on if the resistance is on the intake side or the exhaust side. Try covering the intake and exhaust with your hand and see how it responds when you vary how much of the intake/exhaust is covered by your hand and when you move your hand closer and further away from the fan.