The largest ETF tracking high-yield corporate bonds was down Wednesday for the fourth straight session from a multiyear high, unnerving bullish stock investors and those who have piled into junk debt funds recently.

The $16.3 billion iShares iBoxx High Yield Corporate Bond (NYSEArca: HYG) slipped 0.5% before the Federal Reserve statement while the $13 billion SPDR Barclays High Yield Bond (NYSEArca: JNK) was down by a similar percentage on above-average trading volume.

Many analysts keep a close on high-yield bonds to gauge risk appetite and the health of credit markets. Weakness in junk debt often presages downdrafts in stocks.

The two largest high-yield ETFs were among the top-selling funds last year. HYG pulled in $4.5 billion in 2012 while JNK added $3 billion in fresh cash as fixed-income investors took on more risk to boost yield in a low-rate market for bonds.

The junk debt ETFs are currently paying 30-day SEC yields of about 5%.

Some investors and advisors are using high-yield bonds as an equity proxy after the financial crisis, said Josh Brown at The Reformed Broker blog in a post earlier this month. “[I]t’s also something I’ve seen other financial advisors doing as a half-way measure toward coaxing their risk-averse clients off the sidelines without putting them into a higher equity weighting,” he wrote. [Are High-Yield Bond ETFs Overvalued After Big Run?]

Investors piling into junk bonds have pushed yields to all-time lows. “Every time junk bonds look like they have no more room to gain, they gain some more,” reports Michael Aneiro at Barron’s.

Still, some analysts point out that high-yield spreads relative to Treasuries aren’t flashing bubble-like conditions at the moment.

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