* China investment, factory output miss expectations
* U.S. Fed to meet Wednesday and Thursday
* Apple up after iPhone pre-orders set to beat record
* Indexes down: Dow 0.49 pct, S&P 0.54 pct, Nasdaq 0.45 pct (Updates prices, adds commentary)
By Sinead Carew
Sept 14 (Reuters) - U.S. stocks were lower on Monday afternoon as some investors stayed on the sidelines ahead of the Federal Reserve's policy meeting this week and others worried about slowing growth in China after weak data.
Stocks are expected to remain volatile before a Federal Reserve announcement scheduled for Thursday after a two-day meeting at which it will decide whether or not to make its first interest rate increase since 2006.
"It's a quiet one today. With the Fed coming out Thursday nobody wants to take any real shots," said Michael Matousek, head trader at U.S. Global Investors Inc in San Antonio.
A broad group of economists polled by Reuters last week bet on a September move by a slim margin; economists at banks that deal directly with the Fed, known as primary dealers, picked December as more likely; and traders of short term interest rate futures were giving a rate rise this week only a one-in-four chance.
Also weighing on stocks Monday was data showing China's investment and factory output in August missed forecasts, raising the chances that China's third-quarter economic growth may dip below 7 percent for the first time since the global crisis.
"China continues to be a concern as investors look for a bottom in regard to the country even though the government has a lot of room to stimulate growth," said Chris Bertelsen, chief investment officer of Global Financial Private Capital in Sarasota, Florida.
At 2:33 p.m., the Dow Jones industrial average fell 80.57 points, or 0.49 percent, to 16,352.52, the S&P 500 lost 10.55 points, or 0.54 percent, to 1,950.5 and the Nasdaq Composite dropped 21.89 points, or 0.45 percent, to 4,800.45.
Nine of the 10 major S&P sectors were lower led by the materials index's 1.18 percent fall. The utilities sector was up 0.14 percent while the energy index fell 1 percent as oil prices fell 1.6 percent.
Stocks have been volatile since China devalued its currency in August. The S&P 500 has had moves of at least 1 percent in more than 10 sessions since Aug. 20.
Last week the S&P 500 and Nasdaq Composite posted their biggest weekly gain since July.
The Fed has said it will raise rates when it sees a sustained economic recovery with emphasis on jobs and inflation.
Apple shares added 1 percent at $115.37 after it said iPhone pre-orders were on track to beat last year's first-weekend record.
Solera Holdings, jumped 8.6 percent to $53.70 after it agreed to be acquired by an affiliate of private equity firm Vista Equity Partners for $3.74 billion.
Declining issues outnumbered advancing ones on the NYSE by 2,086 to 885, for a 2.36-to-1 ratio; on the Nasdaq, 1,721 issues fell and 1,020 advanced for a 1.69-to-1, ratio favoring decliners.
The S&P 500 posted 2 new 52-week highs and 5 new lows; the Nasdaq recorded 39 new highs and 63 new lows. (Reporting by Tanya Agrawal; Editing by Nick Zieminski)