I agree with the others that this sounds like AF assist.... My 40D used to do this to me at times. The 430 EX II and 580 EX II have the (almost) IR focus assist which projects a series of lines on your subject and provides high contrast areas to assist with the AF. Works MUCH better than strobing flashes as it provides a series of lines which will provide good contrast even on a white/neutral wall. AF will work MUCH better (a VERY good reason for my 580!)...