The unbridled enthusiasm for tech stocks is waning, perhaps offering up a clue that the broader market is due for a breather. 

The NYFANG Index, which measures the performance of top tech names such as Facebook (FB) - Get Report , Amazon (AMZN) - Get Report , Netflix (NFLX) - Get Report and Alphabet (GOOGL) - Get Report , has rallied about 8% in May as concerns have eased over data privacy and over-valuation. Netflix shares have led the charge, surging a cool 13% -- the move has catapulted Netflix beyond Disney (DIS) - Get Report as the most valuable media company

All told, the NYFANG Index is only 3% away from its mid-February all-time high. 

But before investors pop the champagne corks, it would be wise to keep one thing in mind.

The latest march higher for the NYFANG Index has happened on declining volume (see chart below). That could be viewed as a sign investors are growing worried about a more volatile broader market and what it means to high growth sectors trading at inflated multiples, such as big-cap tech. 

Where is the volume?

To be sure, investors have good reasons to pull some money off the table. TS Lombard strategist Dario Perkins points to four areas of concern:

  • Eurozone data that has missed consensus forecasts by the widest margin since the global financial crisis. Perkins calls EU macro data in 2017 "implausibly high", so a slowdown was inevitable.
  • The OECD's leading indicators have dipped.
  • Global industrial orders and trade data have softened, notably among Asian supply chains.
  • Emerging markets more broadly have slowed, in part thanks to the stronger U.S. dollar.

Facebook, Amazon and Alphabet are holdings in Jim Cramer's Action Alerts PLUS. Find out why here.