Crackling noise on my Vox playbacks

Hi everybody. I’m new on this site. Upgraded my cubase recently from AI4 to 6 on windows 7-32bits. Recording as usual without peaking, but during playbacks I often hear some disturbing crackling sounds espcially from the vocal channels. The VST channels playbacks run smoothly. I’ve tried setting the latency levels to high, low and even normal on my Yamaha USD ASIO sound card, the crackling sounds woun’t just go. Never had this problem on AI4. Could anyone with better knowledge of this program please advise? Much appreciation.

Is the correct ASIO driver selected in device setup? Are the channels clipping? Is Control Room as well as your Main Outs enabled at the same time?