* AutoZone edges up, results show profit growth
* Carnival slides, hurt by downgrades, forecast of loss
* Facebook rises after brokerage upgrades
* Major indexes down less than 0.1 percent
NEW YORK, Sept 25 (Reuters) - U.S. stocks dipped in choppy trading on Wednesday, with the S&P 500 eyeing a fifth day of losses, as a lack of progress in budget negotiations in Washington puzzled investors.
Concerns over a potential U.S. government shutdown added to mixed signals on the immediate future of the monetary policy that has given support to equities, keeping traders on edge. Still, the possibility of a last-minute deal in Congress eased some selling pressure brought by the uncertainty.
"The government is creating uncertainty, weighing somewhat on the markets," said Doug Cote, chief market strategist at ING U.S. Investment Management in New York.
"I see some volatility over the next month due to our democratic process and politics, but once it is clear, this market is going higher," he said.
Most Republicans shunned Texan Senator Ted Cruz's marathon diatribe against President Barack Obama's health insurance reform law, which was delaying Senate consideration of a stop-gap funding measure needed to avoid a government shutdown in six days.
Adding to the drama, U.S. Treasury Secretary Jack Lew warned Congress that the United States would exhaust its borrowing capacity by Oct. 17.
The Dow Jones industrial average fell 5.46 points, or 0.04 percent, to 15,329.13, the S&P 500 lost 1.17 points, or 0.07 percent, to 1,696.25 and the Nasdaq Composite dropped 1.958 points, or 0.05 percent, to 3,766.296.
If the S&P ends the day lower, it will have dropped for five straight days for the first time this year.
Data showed orders for long-lasting U.S. manufactured goods barely grew in August, in a possible sign that companies are holding back on investments due to uncertainty over government spending. A separate report showed sales of new single-family homes rose in August but held near their lowest levels this year, as a sharp rise in interest rates weighed on the U.S. economy.
Facebook shares rose 1.5 percent to $49.18, brushing against $50 after a slew of brokerage upgrades.
Medical device maker Stryker Corp will buy smaller peer Mako Surgical for about $1.65 billion to gain access to Mako's technology for robot-assisted orthopedic surgery. Mako shares surged 82.3 percent to $29.48.
Carnival Corp shares fell 4.6 percent to $32.94 after several brokerage downgrades in the wake of the company's warning it could report an adjusted loss for the current quarter.