hear a popping noise when wheels turned all the way

every once in a while I hear a slight popping noise when I turn the wheels all the way to the left, not sure about the right but I don't know what that is.  I don't think it is cv joints because it is definitely not a loud popping noise and doesn't happen all of the time.  I just want to know what it could possibly be.  Someone suggested that it may be a U joint, whatever that is.  It is on a 1987 BMW 325i.  Car is in excellent shape and I would like to have it looked at and not necessarily worked on depending on the cost of whatever it is to fix.  On a fixed income so I would really just like to know what the possibilities are as to what it is that it could be.