it's a continuous pop. if it were a valve problem...wouldn't it pop at idle and higher rpms? it only makes the popping noise if the throttle is opened quickly.
If it pops continuously it's likely a valve train issue and the reason you don't hear it at idle is the carb blades are shut muffling it. Not sure what to say on why your not hearing it if you accelerate slowly. Back in the early 80's I replaced a lot of cams on 305 Chevy's that had the soft cam issue. That's the way they acted when a lobe went flat but any time a valve is stuck shut it will do that.
