Remove audible noise generated by buck converter
The ISP452 is limiting the driver PWM signal frequency to max 4 kHz.
  Any other suggestion?


Try using a P channel MOSFET and switch at an inaudible frequency i.e 50 kHz. Not only will you get the benefit of silent running but your inductor can be much smaller (and cheaper) for the same output ripple voltage: -



If your PWM drive signal is at 5V logic then one further transistor will be needed to perform level shifting like this: -