Sharp noisy sounds

I just bought 1994 model GLI Toyota Corolla seven months ago with 92,000 kilometers. When I test drove the car in the heavy traffic streets with a maximum speed of 60 kph rpm, the car seemed all okay and the engine sounds in good running condition. Lately however, when I went to our province with a wide road with less traffic when I managed a high speed at 8o kph, I noticed a mosquito sound in front of the dash board. When I press the accelerator further to gain more speed, the sharp sound became louder. And at the speed of 100 kilometer per hour, the sound turned very louder and of many origin from high pitch to midrange, enough to scare my sisters on board. I notice that sounds travels from one side to the other. But when my speed reduces, the noise gradually lessened and completely gone at the speed of 6o kph.
I have the car checked by several mechanics. The first one suggested to remove the alarm/warning device of the car, but he never locate any alarm or warning device. I went to another mechanic and he suggested to replace the brake pads and place grace on the bearings, but just the same the problem was not solved. The next repair shop suggested to unplugged the speedometer while in full speed, the noisy sounds still there. Another mechanic tried to place on neutral while at 100 kph and turned off the power and stopped the engine the same full speed but still the same the the noisy sounds were not removed. One time I have the car hanged on its front wheel and rev it at a speed from 80 kph to 200 rpm. There are no sounds when the car does not run because it was hanged but the rpm reached to 120 to 200rpm. Could you please tell me what causes the sharp sounds near the dash board when my speed reach 80 kph and gets louder as the speed increases. Thanks and more power.