Tom, you hit the nail on the head, and BTW, thanks for a great description of what I have warned against in several posts. Venting for other purposes aside, you need a minimum of 1 sq. ft. of free vent area (screens and louvers cut down free area) per 1000 CFM of air to be moved. This assumes that the fan can generate about 0.1 inch of water of static pressure. Even at this level of vent area there will be significant pressure in the attic so more is better. You will always have some pressuriaztion, you just want to get it down to reasonable levels. Once you do, the fan will quiet down and the house will be cooler.
As a parting note, you have just had a graphic demonstration of all the air connections between an average attic and the house and basement. Realize that next winter, the process will reverse, and the air which you have paid to heat will begin leaving via those connections, along with any moisture from the basement. Depending on where you live, that translates that to high heating costs, ice dams, etc. You might want to browse around at www.weatherization.com, once you set your fan to rights.