For a point of troubleshooting, what are the possible causes of loud pinging noises in wheels? My first thought is that the wheel hub would need overhauled or checked over. Are there any other causes?
Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Metallic pings are usually spokes settling back in to place after truing. Hub noises tend to be more of the grinding/gravely variety. Spokes pinging should stop pretty quickly once the spokes all settle though. If the sound goes on, something else is happening. It's a little hard to diagnose something from a verbal description of a sound. I'd take your wheel off and turn the axle by hand, is it smooth? Is it loose (side-to-side play in the axle. Are there any loose spokes? Is the wheel straight? These will start ruling out issues that can cause sounds.
I went on a couple of pretty long rides (for me anyway) in the last three days, and I noticed the wheel sounds were very loud but settled after I got into the ride a while. So this would seem to be correct, though I noticed the sounds repeated at the beginning of the second ride...so nothing to worry about?
For the length of rides I'm starting to take, I guess I'm starting to worry a lot more about maintenance now...
Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?
Something to worry about. If it was just spokes settling, it wouldn't be coming back like that. I would do the simple axle checks I describe above and check for loose spokes. But it sounds like something is going on that's only going to get worse.