The Nasdaq Composite Index has been on a tear lately, soaring 19 percent since Oct. 16 to a 15-year high.

That represents "a little bit of a bubble" for some technology stocks, Bill Gross, manager of the $1.5 billion Janus Global Unconstrained Bond Fund, told CNBC.

Tech stocks account for 43 percent of the Nasdaq.

The index stood at 5,007 Monday afternoon, close to its March 2000 record of 5,131.52. A year after reaching that peak, the Nasdaq had plunged 60 percent amid the dot-com crash.

While this time around is different than 2000, there's "certainly an overvaluation that might, at some point, be corrected," Gross said.

On the bond front, Gross said negative yields, which are now present in some countries, spark bubbles and distort capitalism.

The sub-zero rates "confiscate capital" and push investors mindlessly into stocks, he said. "There is a translation mechanism that the central banks are willing to endure," as they ease policy around the world, Gross said.

As for the Nasdaq, not everyone thinks it's overvalued. "We're certainly not in bubble territory,'' James Stack, head of Stack Financial Management and editor of the newsletter Investech Research, told USA Today.

While the six-year-old bull market is in the late innings, the Nasdaq could advance to 5,300 or even 5,500 before the party ends, he said.

The index' valuation is much more reasonable now than 15 years ago. The Nasdaq currently has a trailing price-earnings ratio of 23, compared to a stratospheric 120 in March 2000, according to The Wall Street Journal.

"A grounded rally is probably a decent way to put it," Hank Hermann, CEO of Waddell & Reed Financial, which manages $125 billion, told the paper. "There’s certainly not anything bubble-ish going on in valuations of the big-cap technology stocks."

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