I have a 1988 Ford Thunderbird with 68,000 miles on it. I've had the weirdest noise for a long time and now it's getting louder. I don't hear it very much while I'm driving, but when sitting at a light with the radio off I hear this buzzing noise coming out of the trunk.
It's probably the fuel pump. The majority of fuel pumps used on fuel-injected engines are located in the back of the vehicle at the bottom of the fuel tank, submerged under gasoline. These small electric-motor driven pumps send fuel directly from the tank to the injectors located atop the engine. These pumps are capable of supplying large amounts of fuel pressure and volume. A device called the fuel pressure regulator, placed in-line just after the injectors, holds back the needed pressure and bleeds off the remainder back to the tank by means of a return line.
Throttle body injection needs about 10-15psi, while port fuel injection requires more like 30-50psi. One reason behind keeping the pump in the tank is to maintain fuel pressure from the fuel tank all the way to the engine. This eliminates a problem called "vapor lock". This is the actual boiling of fuel inside the fuel lines. Vapor lock may cause bogging, chugging, surging and just about anything else that you don't want your engine to do. The principle is the higher the pressure the higher the boiling point of a liquid.
Any pump of this style is going to hum, buzz, whine or whatever you want to call it, a little bit. However, if the noise is noticeable from inside the car, there is a problem, and you may be losing fuel pressure. Sometimes a clogged fuel filter can put enough strain on a pump to cause excessive noise and, eventually, permanent damage.
In your situation, after hearing the noise for a long period time, you're most likely going to need a new pump and filter. Sometimes a noisy pump sounds like a fish flopping around at the bottom of the tank.