Engines tend to run richer in the air. Could be foaming caused at the higher speeds. Is your gas tank well padded and insulated from the airframe? I've actually never run into this with a gas engine, so I don't know that gas/oil mix foams like glow fuel does. Might want to ask this of the Gas Engines forum here.

Also, sound travels about 600 fps so if your plane is 1,200 feet away it takes two seconds for the sound of the throttle change to reach your ears.