Never overlook even the smallest problem with your vehicle's transmission, as a small problem can soon turn into major, very expensive repair work. That small problem can also cause your car's transmission to seize up or fail outright, which is obviously very dangerous when on the road, and very inconvenient at any other time. Note two of the more common transmission problems your car might be experiencing, so you know what you might be facing by way of repairs and know what to discuss with your mechanic.
If your hear your car making a clunking noise, the first thing you can do is note the location of the sound. If it's coming from around the tyres or backside of the car, this might be a problem with the differentials, which are part of the steering and not actually the transmission. These are gears that are located outside the wheel well and which allow the tyres to rotate at different speeds, which is needed when you turn a corner. If these gears wear out, they clunk rather than mesh, and you may feel some pulling or resistance from the tyres.
If the clunking sound is coming from under the hood and not near the tyres, this often means the gears of the transmission are wearing out. They will also clunk into place rather than mesh; the clunk may even be following by a whirring or squealing sound, as the gears turn independently rather than with each other. This is a sure sign that the teeth of the gears are worn and they need replacing, and this should be done quickly or the transmission may outright fail.
If you change gears of your car and you hear a grinding sound, and you feel resistance from the car as you try to move forward or backward, this can mean that the transmission fluid is low or contaminated. The teeth of those gears are meshing together but not moving very easily, so they grind or rub against each other as the gears spin. In turn, you hear this sound and feel resistance to your car's movement.
Adding new fluid may not be a solution, as old and contaminated fluid can be circulating debris around those gears and causing more damage. A transmission flush, which empties the transmission completely before filling it with fresh fluid, can be the better choice. This is especially needed if your car is very old, as this means the transmission fluid is also likely to be old and contaminated and not working to keep those gears lubricated properly.