I've been getting them when OC-ing too high. When that happens I usually throttle back 5MHz or until they disappear. Although probably not damaging to your GPU they are unpleasant when working on the PC. Also, considering that too high settings can cause hardware errors in your miner which equal to wasted work and shares, I would throttle back just a bit to ensure everything runs smooth.
One other suggestion that I came across the forums and seems to work wonders is disabling the 'bling' in windows. To do this, go to Control Panel and type in "windows performance" in the search field. It should show an entry under system for "Adjust the appearance and performance of Windows". Click that and select "Adjust for best performance". It got me about 40MHs extra on my 7870s.