Toyota Repair: overdrive, electroniccontrol transmission, 1990 toyota camry, electric solenoids

Question
Could you please explain to me what the overdrive and the ect(electronic control transmission) is for? Also my 1990 toyota camry sounds very loud in the front of the car. Kind of like a rumbling, is this normal with overdrive. Any suggestion as to where to take it to be looked at?
thank you
teresa

Answer
ECT(electronic controlled transmission) is designed to use electric solenoids in the transmission controlled by a computer this controls the upshift and downshift operation of the transmission, overdrive is actually a fourth gear that lets the engine revolve at a lower speed than a direct 1:1 gear ratio that is common in three speed automatic transmissions, this tends to result in better gas mileage and less wear on the engine with higher roadspeeds.
Any kind of rumbling noise is not normal and I suggest that you have a Toyota dealer diagnose the problem.