Denis,
if you try to sample a synthesized sound (like a synth lead) you should choose a starting point for the loop AFTER the decay phase of the sound, when the steady phase begins. Unless there is a heavy vibrato, the sound should then be uniform; otherwise try to set on your synth the vibrato speed and depth to zero.
A short loop should do the trick; set the loop end point about one second apart from the starting point and then keep pressing the "find loop" button until you are satisfied with the result. If there is still a "pop" or "click" then try to apply some crossfade transition to make the noise unaudible.
If, on the other hand, you cannot get rid of the vibrato, then choose a loop that matches the vibrato wave.
The final step is to cut the sample shortly after the end of the loop, to save memory and hard disk space.
Like for everything else, experience leads to perfection.

[This message has been edited by Dreamer (edited 01-07-2002).]