I beg to differ (respectfully) with everyone except red58.
At any given RPM (1500 in this example) the fuel/air mixture charge volume would not change, regardless of load on the engine. In a naturally aspirated engine you can't put any more fuel/air into the cylinder than what is physically drawn in by the piston traveling downward while the intake valve is open. The change in sound is a product of an engine trying to do something which the resistance placed upon the drivetrain is preventing it from doing.
No different than stepping on the gas while standing on the brakes.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission