Coming back from a nice camping trip, I noticed all air pumps made higher and higher pitch when the air bed were about to be fully inflated. It is heard like the the motor is speeding up when it is overloaded. I opened the pump but it seems very simple - electricity from the batteries are directly feed into a motor through a switch. However, my understanding about a electronic motor is that the speed should decrease when load increases. Is there anything I missed?
Finally, I made my poor man's power supply by a computer PSU and went through a test. Driven by 3.3V, the air pump draws 4.5A when it runs with a free airway; when the output nozzle is blocked, the current drops to 3.3A. The test supports Dave's answer and I agree that this question is more related to physics.
The picture is the test setup.