* Bullard says stimulus could wind down from October
* Trading volume heavier than normal on quadruple witching
* FireEye, Rocket Fuel shares make their trading debut
* Both open more than 100 pct above IPO price
* Dow off 0.3 pct; S&P off 0.2 pct; Nasdaq up 0.1 pct
By Angela Moon
NEW YORK, Sept 20 (Reuters) - U.S. stocks edged slightly lower on Friday following comments by a senior Federal Reserve official that sparked concerns that the central bank's stimulus efforts could be scaled back from October.
Trading volume was heavier than usual as Friday marks the "quadruple witching," when stock index futures, stock index options, stock options and single stock futures all expire on the same day. Trading increases as investors replace or repurchase existing contracts.
In addition, all three major U.S. stock indexes, as well as the FTSE and U.S. index trackers, will rebalance their portfolios to match revisions to index weights. This happens at the close of trading, which could push trading volume higher as transactions jump at the exact close.
"Since the largest of these products which are index futures and options expire on the open, the effect is most pronounced at the open. However, with stock and ETF expirations, as well as index rebalancing, occurring at the close, there can also be significant impact on the close," said Ana Avramovic, derivatives Strategist at Credit Suisse in New York, in a note to clients.
Investors waited to see if speeches by several U.S. Federal Reserve officials later in the day would shed light on the Fed's surprise decision Wednesday to maintain stimulus for now and how much longer the program was likely to continue.
"The Fed wants everyone to speak their minds to show that the governors, officials within the Fed all have differing opinions, that there isn't a unifying voice," said Timothy Ghriskey, co-founder of Solaris Asset Management in Bedford Hills, New York.
"They want the uncertainty because they don't want the market to be surprised by anything. They want the financial markets to ease into change."
The Nasdaq was supported by shares of Apple Inc, which rose 0.6 percent to $474.91 as the company's latest version of its iPhone product line hit stores around the world.
The Dow Jones industrial average was down 39.24 points, or 0.25 percent, at 15,597.31. The Standard & Poor's 500 Index was down 3.05 points, or 0.18 percent, at 1,719.29. The Nasdaq Composite Index was up 1.84 points, or 0.05 percent, at 3,791.22.
Helped by a rally that took the Dow and the S&P 500 to record highs on Wednesday following the Fed's decision, both indexes are now on track for a third consecutive week of gains.
In company news, Cybersecurity company FireEye Inc shares opened 101.5 percent above its initial public offering price at $40.30.
Shares of advertisement technology company Rocket Fuel Inc also more than doubled in their trading debut, valuing the company at more than $2 billion, a day after it priced its IPO at the top end of its revised price range. The stock opened up 106.7 percent above its IPO price at $59.95.
In an interview on Bloomberg TV early Friday, St. Louis Federal Reserve Bank President James Bullard said a wind-down of the Fed's $85-billion monthly bond purchase program from October was possible. On Wednesday, the Fed surprised economists and investors with its decision to delay scaling back stimulus from September.
Several senior Federal Reserve officials are scheduled to speak Friday at various events.
Bullard speaks on monetary policy to the New York Association for Business Economics at 12:55 p.m. EDT (1655 GMT).
Just ahead of that, Kansas City Federal Reserve Bank President Esther George delivers a speech on the economy to the Shadow Open Market Committee in New York at 12:30 p.m.
Federal Reserve Governor Daniel Tarullo speaks on macroprudential regulation in New Haven, Connecticut at 12:40 p.m. EDT.
Minneapolis Federal Reserve Bank President Narayana Kocherlakota talks at a conference on risk in options prices in New York at 1:45 p.m. EDT.
In Washington, Republicans in the U.S. House of Representatives were expected to vote for a bill to gut President Barack Obama's healthcare law while temporarily funding other government programs, ignoring a warning from the White House that the measure would be vetoed.