UPDATE 1-Investors pull $6.4 bln from U.S. stock funds ahead of Fed speech

(New throughout; adds data about mutual fund and ETF flows, analyst quote, table, byline) By Trevor Hunnicutt NEW YORK, Aug 25 (Reuters) - Investors trimmed risk assets before a gathering of central bankers on Friday, pulling $6.4 billion from U.S.-based stock funds in a course-reversal during the week ended Aug. 24, data from Lipper showed on Thursday. The outflows, which came a week after cash poured into stock funds for the first time in five weeks, are the last data showing U.