I have a 2007 IS250 and don't recall any fan noise or squeak. However I hear (feel) a pressure buffeting effect sometime with the fan is on. This happens for any of the HVAC settings that I have used.
I have also heard or felt the same thing is a loner IS250 so I think it is just inherent in the design.
What is your noise like?
Regards,
I'd love to imitate the sound for you but I can't. But like you said, it sounds like "a pressure buffeting effect" and it's a rapid sound. I don't recall hearing the sound when I test drove the other IS. I also had a ES as a loaner car and that didn't have it either. So, I was just wondering...
I notice a little thumpy thumpy maybe pressure sound sometimes if I have the fan running with the engine off. One thing you might want to check is the cabin filter. If it's dirty there will be extra load on the fan.