* All three major indexes on track to end week higher
* Twitter jumps on report company moving closer to sale
* Indexes down: Dow 0.24 pct, S&P 0.21 pct, Nasdaq 0.24 pct (Adds details, changes comment, updates prices)
By Tanya Agrawal
Sept 23 (Reuters) - Wall Street was lower in late morning trading on Friday as investors assess valuations after a three-day rally spurred by optimism that the Federal Reserve will hold off from raising interest rates in the near term.
Some analysts have been questioning valuations as corporate earnings continued to be tepid. The S&P 500 is trading 17.4 times expected earnings, above its 10-year average of 14.7, according to Starmine.
"The market is taking a bit of a breather after a strong week," said Mike Bailey, director of research at FBB Capital Partner, in Bethesda, Maryland.
"Investors are saying markets are already expensive and they've become more expensive this week, so this is a bit of a reversal."
Bailey added that if earnings for S&P companies in 2017 were flat or lower, that would be a major cause for concern.
The S&P 500 index notched its best two-day performance in more than two months on Thursday and is up about 1.5 percent this week.
Fed Chair Janet Yellen said on Wednesday that U.S. growth was looking stronger and rate increases would be needed to keep the economy from overheating and fueling high inflation. But the central bank maintained the low-interest rate environment that has helped underpin the bull market for stocks.
The U.S. central bank had hinted that it might raise rates before the year ends and interest rate futures were pricing in roughly a 60 percent chance of a rate increase by December.
At 11:06 a.m. ET (1506 GMT) the Dow Jones industrial average was down 43.42 points, or 0.24 percent, at 18,349.04, the S&P 500 was down 4.57 points, or 0.21 percent, at 2,172.61 and the Nasdaq Composite was down 12.87 points, or 0.24 percent, at 5,326.66.
Six of the 11 major S&P sectors were lower, with the technology index's 0.64 percent fall leading the decliners.
Twitter shares jumped as much as 23 percent to $22.89, posting its biggest one-day percentage gain since its market debut in 2013, after sources said the microblogger has initiated talks to explore a sale.
CNBC earlier reported suitors for Twitter could include Alphabet's Google and Salesforce.com. Salesforce was down 3.6 percent at $71.87 and Alphabet was down 0.14 percent at $818.80.
Facebook was down 1.5 percent at $128.11 after the WSJ reported the social media giant overestimated viewing time for video ads by 60-80 percent for two years.
Bats Global Markets jumped 19.6 percent to $31.85, a day after Bloomberg reported CBOE Holdings is in talks to buy the stock exchange. CBOE was up 1.5 percent at $70.47.
Declining issues outnumbered advancing ones on the NYSE by 1,680 to 1,132. On the Nasdaq, 1,437 issues fell and 1,164 advanced.
The S&P 500 index showed seven new 52-week highs and no new lows, while the Nasdaq recorded 49 new highs and 10 new lows (Reporting by Tanya Agrawal; Editing by Don Sebastian)