Had a clicking noise that was not consistent. Only occured during pedaling (sitting or standing). Went nuts trying to find problem. Also have a carbon frame which seems to propagate sounds more so than other frames (or so I have heard?). Checked all the usual stuff. Finally while looking at wheel, noticed that non-drive side radial spokes were somewhat loose. Tightened them all up, and noises gone! I felt pretty dumb not noticing this before. So this leads to question of what should proper tension be, and should I use some thread treatment to keep them from loosening up again.
Two ways to check tension, by sound or with a Park TM1 tensionmeter. The sound method simply involves tapping with your fingernail a couple of spokes you know were good, then go to the loose ones and do the same, then simply adjust the spoke tighter if the sound is dull or low or looser if the sound is higher. The tool cost about 57 dollars. Is the tool worth it? If your building wheels for a living yes, but otherwise no. I never had that tool, in fact don't even own a truing tool, and haven't had the need in over 40 years of messin with bikes. I simply use the brake calipers to true the wheels by closing down the calipers till they just barely miss the rim, then spin the wheel till a caliper hits the rim then adjust accordingly. Tension wise I just hit the spoke with my finger nail and listen for the sound, and you can tell by spoke movement where two spokes cross. See this for more detail: http://www.bikexprt.com/bicycle/tension.htm
Wheel is still centered. I only tightened them up 1/2 turn. I may be able to do a little more. I think they had loosened up enough to pull the wheel slightly toward the drive side. I think it would take significantly more tightening to make a significant change in centering.
Most of the wheel building guides talk about setting the tension on the drive side, not to worry about left side; having the wheel centered will take care of it. It is always the case that the left side spokes are slightly looser.
It is also true that radial spokes tend to loosen up.
And I like the idea of just listening to the sound when plucking the spokes to get an idea of tension.