It seems a little fast but for the moment I'm going to assume 3500 rpm is normal at that speed in high gear. What type of noise are you hearing? Rumbling, roaring, whining, squealing, knocking? Can you only hear it or can you feel it as well? If so, where do you feel it? In the steering wheel, the pedals, the shifter, the seat?
What it sounds like to me is a harmonic vibration. These vibrations are always present when you have something that is spinning rapidly i.e. engine, transmission, wheels, axles, etc. I would be looking at the exhaust system for cracks in the pipe, or heat shields, or possibly the catalytic converter. With the high mileage on the vehicle, it may have a cracked bracket on an engine mount causing normal engine vibration to transmit through the body. Unfortunately, the sources of these types of noises are sometimes very difficult to find and you may have to take it to a mechanic that is good in noise and vibration diagnosis. It is very unlikely I will be able to determine the source without driving the vehicle myself.