why does my LED fixture buzz when the dimmer switch is instaled, and no buzz when i reinstall the SPST original wall switch and how can i stop it ?

We are sorry to hear about the issue you are having with this application. Unfortunately, there is currently no industry standard when it comes to manufacturing LEDs and the performance will vary greatly from one to the next. When LEDs are used on a dimming control, depending on the design of the bulb, they can produce high current spikes. These high spikes may cause the bulbs or the dimmer itself to produce an audible buzz or hum. Due to design and manufacturing variation, the volume of this buzz will differ from one to the next. We can suggest choosing a different brand of bulb or contacting the LED manufacturer to see if they have a dimmer that they can recommend which will not produce an audible noise.       If you visit www.lutron.com/ledtool you will be able to generate a list of compatible LED products for our dimming controls. This list will link you to report cards which will list the performance you will experience with our controls and the LED’s listed. If buzzing is noticed during testing it will be listed at the bottom of the page under “Test Comments”.