[wmjosiah]

After reading up on this a bunch more, I came to the conclusion that there are a lot of valves in the system that can get stuck. Hm. Stuck solonoid valves gummed up with who knows what in such a way that causes them to suck air into the intake... OK, let's spray some throttle-body/intake system cleaner in there. Did that, took it for a drive, a bit later, when it usually makes the noise, I could clearly hear it open, suck the liquid into the system, and then... no more noise. I drove around for five more minutes and heard it open one more time, much smaller noise. Stopped, sprayed some more in, drove around for another 10 minutes, no more noise. This may, obviously, not be a permanent fix. We'll see.