Are you sure it is the transmission that is clunking? It could be the u-joints on the drive shaft that are making the noise. When the u-joints on the drive shaft are bad, there is an audble clunk when you accelerate and when slowing down (basically the same motions you go through when shifting gears).
A clunking sound on shifting gears and while driving can be indicative of failed transmission mount. If the condition is always occurring regardless of shift and driving on level surfaces an internal automatic transmission mechanical condition is possible also.
You need to use the clutch and the shifter smoothly together!
When you have problem shifting gears, you transmission has a problem.
Manual transmission is when you use the shift stick to shift to different gears in a car instead of driving without shifting gears,which is auto transmission.
Shifting forks.They slide the gears along splines engaging them to another gear.
Your transmission will slip and clunk or bang when changing gears, can have trouble changing thru the rand of gears, ans will eventually over heat and burn out.
If the 1997 Nissan Altima with an automatic transmission is shifting gears too early, it could be that the shift solenoid for the transmission is faulty. Both the shift solenoid and the speed sensor on the transmission should be checked.
It could be that the transmission fluid is low, or it could be that mechanical parts have worn down. If so, it may be time for a replacement.
broken transmission mount there is a front left, rear left and a center mount
Check the transmission fluid level, be sure it is at the full line. A worn out universal joint could be the source of the clunking noise when the transmission is shifting.
try the transmission ouput speed sensor
An automatic transmission might be slow shifting from first to second because of low transmission fluid. You might also have damaged the gears in the transmission at the area between first and second.