On my ride last Saturday, I started hearing repeated noises that sounded like spokes pinging.

The noises didn't happen all the time, they seemed to start when I was descending, and I don't recall hearing them on flats or climbs. They came in an irregular sequence, in twos and threes. In total, during the two hour ride I probably heard fifty or seventy such noises. Didn't seem synchronized to wheel or crank rotation. Both rims were and remained true. I eyeballed them during the ride, and checked them carefully after the ride. By "true" I mean only about 1 mm runout.

Yesterday I checked the rims again, made them even truer. I run my pads as close to the rim as possible. During the ride, I had maybe 3/4" brake lever tip free play, now I have maybe 1/2". I'm not seeing anything else wrong with the bike. It is the bike pictured below. I built the wheels a few years ago, have ridden them a fair bit since, never had any problems.

I'll go for another ride tomorrow and see if the noises come back. Any idea what they might have been? Is there something else on a bike that makes pinging noises? Can spokes loosen enough to make so many pinging noises, while the rims remain true?