The U.S. economy is growing at a fast clip, and the bull market is entering its ninth year. But some economists are starting worry over rising interest rates and a negative signal from the bond market called a "flattening yield curve."

"The yield curve—with one exception in 1966—has basically predicted every recession," Natixis Chief Economist Joseph LaVorgna told CNBC in July. "If the curve inverts let's say in October, history would say the earliest you'd have a recession would be next summer, next August. And the latest would be August of 2020."

"Now it's possible that this time is different and the curve might be sending a different signal because long rates are relatively low," LaVorgna added.

In July, the U.S. economic growth rate hit 4.1 percent for the second quarter, its fastest pace in four years. And the stock market is poised to make history Wednesday, when the nine-year bull market is supposed to become the longest in U.S. history. Here's what 10 economic experts are saying of when and why the U.S. may hit the next recession.

