I took it back to the mechanic - he called it a spark knock, and told me to run through a tank or two of premium, and then bring it back to him if it was still doing it. He also adjusted the timing on the distributor, but that didn't really make much difference.

He said that it wasn't really anything to be concerned about, apart from it just being an obnoxious noise. I'm sure he'll check the timing belt if it's still pinging after using premium for a bit.

I'm still concerned about wear/damage to the engine, though, and to have to use premium all the time, whether I want to or not, would be a pain.