How to eliminate the buzz noise from a sound system when turned on?

First you need to identify the buzz, Is it a hum, about 120 hz. or is it a higher frequency than this.

Have you added any new electrical devices on the same circuit that could be injecting noise into your system. eg new light dimming or control equipment. Are you power cords and electrical outlet grounds good. How old is the equipment. Filter caps in the power supplies can age and become less effective. Is some other piece of interconnected equipment causing the problem eg turntable connected to amp problem in turn table not amp for example. Oh and a few more. Are you in a townhouse, apartment building, link home etc,, could the neighbours have added something.

Start with the grounding, check all cables and interconnects. If you can't seem to find the source. Try taking your gear to a friends house to make sure it follows your equipment. If it does keep looking for equipment issues. If it does not, look for the noise source in your house. eg fluorescent lights, light dimmers, smart devices, etc. Good luck, hope you find your issue

ive tried everything,grounding, cables anfd amps in good working order,tried the pa systems in different venues,clubs,dance halls,reception areas where nothing its plugged in same area, ive tried it in a completly empty building,with nothing around ,and my system its less than a year old,i use a professional bose towers system.theres been only a couple of places we played at that did not have that problem,maybe i need to go back and investigate how this venue is electracly wired,comparing all these p;laces this couple were fairly new

https://www.techhive.com/article/3063590/home-audio/how-to-get-rid-of-hum-and-eliminate-other-noises-from-your-audio-and-video-systems.html

https://www.psaudio.com/ps_how/how-to-find-and-fix-hum/

https://www.techwalla.com/articles/how-do-i-fix-buzzing-speakers

Sergio, did you check if it was a high frequency electrostatic discharge? If you record it and slow it down in playback, You may find static is the cause. Solve for static is a Magnet just like the ones that attach to newer TVs. I am sure that either way, Static or Not, the magnet helps. Otherwise you need a PCM (Pulse Code Modulation) inverter. The Inverter attaches directly to Mix into Each Line and after Recording that Noise and Inverting the PCM you play it back like a mixer, Simultaneous and once you get it Synch'ed you will not believe the Clarity you can Acheive.

Sergio, did you ever solve your issue? I re read your question and your comments If your ground/short the inputs do you still get hum on your speakers? What are you attaching to the system? Mic, turntable other? Check ALL your cables. Static, maybe but that usually sounds different than hum, think higher freq more like hiss.

Have to add read the articles that William posted as well.