The rapid growth of shadow banks in China over the past few years has triggered worries of a lending bubble that could end in a crash.

The shadow banks consist of trust companies, insurance firms, leasing companies, pawnbrokers and other informal lenders. And regulation of them is loose.

Shadow banks "have created the closest thing China has to the culture of Wall Street," according to The Wall Street Journal. "They take risks that traditional banks won't, going so far as to create investment funds for assets like top-shelf liquor and mahogany furniture."

Editor's Note: Trump Says U.S. Losing Economic Power To China, No Longer A Rich Country

Shadow banks represent the part of China's financial sector enjoying the fastest growth. "For years, Beijing has used the sector as a way to test drive market-driven lending practices that aren't officially permitted at traditional banks," The Journal states.

But experts worry that shadow banks are putting the financial system at risk with questionable loans, just as U.S. financial institutions did during the sub-prime mortgage crisis.

And China's central bank indicated Monday that it's going to restrain credit. Also, earlier this month, the People's Bank of China started curtailing funding for the interbank-lending market, which caused rates to soar.

"The bigger the problem becomes, the less manageable it is," Charlene Chu, senior director of Fitch Ratings in Beijing, tells The Journal.

Jim Rickards, senior managing director at Tangent Capital, sees a raft of problems in China's banking system. "It's a giant Ponzi scheme," Rickards tells Yahoo, referring to wealth management products being offered by Chinese trust companies.

"There's a lot to be concerned about."

Editor's Note: Trump Says U.S. Losing Economic Power To China, No Longer A Rich Country