One of two things.

When you depress the throttle the vacuum quickly drops, but is not instantaneous. Therefore the time lag will add some timing on tip-in from the vacuum advance and you would get knock as you tip in under throttle.

Another possibility is the spring load on the vacuum advance is so little that if you are getting only a couple inches of vacuum is adding 1 or 2 degrees timing and is just enough to cause pinging. This would be the case if you keep the throttle fully depressed from 1500rpm and then hear knock as the engine speed increases through the 2300rpm to 3500rpm range where you have noted the problem.

If you don't have an adjustable vacuum advance already, buy one (Crane is nice) and keep adding spring preload until the knock goes away. Then you can have your performance curve and better gas mileage.