While the Standard & Poor's 500 Index remains only 4 percent below its May 22 record high, serious obstacles are about to emerge, says Byron Wien, vice chairman of Blackstone Advisory Partners.

"I think while valuations are still fair, I think the market has made a lot of progress, and I think there's some trouble ahead," he told CNBC.

"The trouble is that profit margins, in my opinion, have peaked and that earnings are going to be disappointing in the second half, and that's what the market is beginning to sense."

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The possibility that the Federal Reserve will taper its quantitative easing (QE) soon represents another problem, Wien says.

"The market is looking for certain kinds of reality," he said. "Tapering is a negative. Disappointment in earnings would be a negative. Evidence that profit margins have peaked is a negative."

To be sure, if the Fed pulls back its QE to, say, $60 billion a month, that's "still an enormous amount of liquidity being poured into the market," Wien said.

The central bank is currently buying $85 billion of Treasurys and mortgage-backed securities a month.

Still, the prospect of tapering already has investors on edge.

“Markets are wrestling with high volatility and changes, which, in my opinion, are disconcerting to a lot of investors,” David Kotok, chief investment officer at Cumberland Advisors, told Bloomberg.

“We went from QE, relied upon and predictable, to mixed messages in most of the capital markets of the world.”

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