Bored while gluing tubulars and taking a break. Checking ad campaigns on other forums and decided to pop back in here and see if there was anything interesting. Skimmed a lot of this but here's my $0.02.

A large number (most) of all inexpensive pre-built wheels used on bikes from OEM's have never been properly stress relieved or tensioned correctly. They were machine built and put on the bike.

Riding them over a period of time can cause non-drive side spokes to loosen/de-tension. you found one that happened to. Just put the nipple back on, take it up to the tension that all of the others on that side seem to be around by plucking it. Stress relieve the wheel (read about it elsewhere), pluck it - get them equal again and then true it.

Do NOT loosen all of the spokes and start over from scratch. For a novice this is a surefire way of assuring you will end up at a shop. FYI - most shops are horrible with wheels. They truly don't know what they're doing but feel like they do.

In general the more you futz with it the worse off you will be. Put the spoke wrench down and walk away. The guy who owned the shop I bought out used to always say, "I loved selling spoke wrenches. I got the money off the sale of the wrench and I was always going to end up with a wheel tension and true job when they came back after they messed their wheel up."

If you stress relieve it correctly you will hear that pinging come out of the wheel. If you're still hearing it when you ride it then hop off and you'll find the wheel is untrue again. Funny how that happens eh? Keep tensioning and truing until the wheel stands true after stress relieving. If it won't stand true and the rim isn't bent then you don't have the right tension balance. Period.