The transmission front cover has a tube the T.O. bearing collar slides on........more then likely the tube has a ridge worn in it that the T.O. bearing collar is getting caught on. This was really common to find in the early 240Z cars. The wear occurs over time when a T.O. collar goes dry and a missing or torn clutch fork dust boot will alow the damage to occur quicker from dirt and water being allowed to enter the bell housing area. A new pressure plate will release in a different area than an older worn pressure plate and this will cause the T.O. collar to operate in a different area of the front cover tube. When the T.O. bearing collar moves into the area of the ridge, it will cause the collar to get stuck for a moment until the pressure forces the collar past the ridge causing a "POP".
