4 clicks per revolution sounds like it is in front of the rear. Indicates a 3:78 rear and sounds like the pin in the shaft coupling pin. I have seen a number of these pins come loose. They slide out as the drive shaft turns and hit the torque tube. The pin is to long to drop out. I doubt this can be repaired by drilling a hole in the tube as someone mentioned. A peice of heater hose is good for listening. G.M.