The blower fan in my Civic is really loud on the high setting; from inside the vehicle, it's much louder than the engine and even louder than wind/road noise at highway speed. Reaching up under the glove box and putting pressure on the middle of its casing while it's running quiets it down quite a bit, so it seems like it's a vibration issue, but I have no idea where to start diagnosing it further. Is this a common problem with solutions, or do I just need to replace the fan if I want to make the noise go away? I suppose I could post an audio recording if that would help.
It sounds like the blower motor has a significant mount of debre jammed into it causeing it be off-balance or possibly even broken a blade. Or, the bearing that it spins on is just worn out. Either way you'll have to pull it down and have a look at what's going on with it, it should be noticable either way. Either it has a significant amount of play in it or there's a few blades missing, or there's an excessive amount of leaves and debre jammed in it.
If it was caused by excess debre, then make sure to have a look at the fresh air filter whenever servicing the vehicle and replace it whenever it gets too dirty.