Is it significant that my bike sometimes makes a "twang" sound when I hit a bump? I recently bought a 2016 Cannondale Synapse Carbon Disc road bike, which is working great -- however I notice that occasionally when my front wheel hits a bump (e.g. a small pothole or the lip of a driveway), the front fork (or something beneath it) goes into some sort of vibration mode and makes a noise that sounds a lot like the "twang" of a door-stopper for about a quarter-second. It's a little surprising when this happens (both due to the unexpected sound and the vibration I feel through the handlebars), but I don't observe any obvious problems other than that. My question is, is this sort of behavior normal/expected with this type of bike, or is it an indicator that something needs adjustment? (The quick-release lever seems to engaged tightly AFAICT, and there is no reflector mounted on the front spokes)

I have a GT Grade Carbon which is designed and produced by the same corporation as your Cannondale. I have the described effect on high speed braking only which is quite common problem and it's easy to find information about it over the internet. Moreover, this effect was one of the reasons manufacturers switched to through-axles on road bikes too, and I believe it makes sense because this vibration makes quick-release loose with time. Make sure to check it more often while you have this effect unresolved.

I wouldn't say that I'm confident on why it happens to you during the normal operation, but some basic sense suggests that it could be something loose, online forums suggest that it could be the headset and I believe it can make sense because that's the only thing keeping your fork steady. I would suggest to you to run through the checks in this video, it's not as complicated as you may think:

Please let us know if tightening the headset helps!