Controllers at LAX often keep airplanes too high for too long while dealing with lower-altitude traffic. If the pilot (and the FMC) has planned for an "optimum" VNAV descent, any delay in the descent will require airbrakes or early flap and/or landing gear extension. The rumble is most noticeable in the 744 only with the last inch or 2 of airbrake lever travel, so many pilots will judiciously use it. Some times, though, we just have to pull it all the way back...