Check the rubber elbow. There may be a crack in the rubber about in the
middle on the bottom(where you cant see). The afm part is attached ridgedly
onto the frame via the air box. The other part is attached to the engine
via the throttle. When the engine is revved, the rubber elbow is flexed.
When the throttle is released, the rubber elbow relaxes. A crack in the
rubber where air can enter can be opened when flexed.You would only see
this when the engine is changing speeds. Also, if you ever use a propane
torch to check for vacuum leaks, air out the area  being checked every few
seconds(as opposed to minutes). Be careful using it near the altenator ,
sparkplugs, and distributor, as there are sparks present in these areas.