Very high-growth, high-multiple stocks “appear frothy” and the boom in special-purpose acquisition companies is one of a number of “signs of unsustainable excess” in the U.S. stock market, strategists including David Kostin wrote in a note Friday. The recent surge in trading volumes of stocks with negative earnings is also at a historical extreme, they said.

However, the aggregate stock market index trades at below-average historical valuations after taking into account Treasury yields, corporate credit and cash, the strategists added.

“Pockets of the market have recently appeared to demonstrate investor behavior consistent with bubble-like sentiment,” the team wrote. “But these excesses present low systemic risk to the broader market given their modest share of market cap.”

With global equities trading around record highs, investors are questioning the potential for future returns amid extended valuations and signs of speculative behavior. The MSCI AC World Index has surged 74% from the March trough last year, with the growth-stock heavy Nasdaq 100 Index up over 90%.

Goldman’s peers at Citigroup Inc. acknowledged stocks worldwide are looking “increasingly frothy,” in separate note Friday, but suggested that valuations continue to lag previous “mega-bubble” periods and risk assets could continue to climb.

“A proper bear market will eventually come, it always does after a bubble,” wrote the Citi team including Robert Buckland. “But markets may get more bubbly first.”

–With assistance from Crystal Tse.