DELL 9020 BUZ sound in speakers and LAG while typing!

DELL 9020 BUZ sound in speakers and LAG while typing! set up 25 of them.  After a reboot the system seems to work fine, no more buzzing sound and the pause while typing for a few days. Then it will start to pausing and buz again..One of my staff found the pause and the Buz sound happen at the same time. I thought it may had been a bad power strip [lack of grounding]...It must be power supply or Motherboard. This does not happen to all 9020 just some but over half of them. I'm now leaning towards Hardware. Any IDEAS