You really should borrow or buy a tension meter if you plan to keep on working on these wheels. It's possible that one undertensioned spoke is making the noise. It's also possible that, if you keep tightening the spokes, you'll overtighten them and ruin your wheel.
Just FYI - Sun CR-18s are, IMO, unpleasant rims to work with. They're hard to get perfectly true vertically... a lot of times there is a hop you just can't get out of the wheel. And they seem to respond to very slight tension changes during lateral truing. They make for a nice strong wheel though.