Why is there a humming sound and the engine revs up and down when you turn on and use the AC in your 1994 Ford Thunderbird?

Ok, the AC takes several horsepower in order to operate. Rather obviously you can't extract energy from nothing, ie.. the AC does not run for free! Turning on the AC causes the drive solienoid to engage the AC clutch which, in turn, puts an added load on to the drive belt. You are hearing the engine speed-up to counter the engagement of the AC clutch, so as not to kill the engine. You will hear the noise generated from the AC compressor when it is engaged. you will probably hear noise, from time to time, from the fan belt...the serpentine belt which runs all over the place in front of the engine and drives the radiator cooling fan...at least in most cases...but you could have an electric fan or both. In any event, this too drives the AC compressor. Belts sometimes squeel because they become dirty, uncommon with modern vehicle like a 1994 model, and they needed to cleaned. It is a good idea to give the belt pulleys some lubrication from time to time as this will prolong the life of the bearings and you won't have little strange noises generated that seem to come and go. Hope this is helpful, and don't worry so much about the bizzare little noises which sometimes are generated from under the hood. If they become loud then you need to do something to find and correct the problem. Vehicles are alot like people in that they have their various quirks, and in that failures are usually the result of many combinded failures that add up to one major heart attack! Change your oil every 3,000 miles with a new filter and you will be motoring along for a good 200 thousand miles easily.