So far, investors are shrugging as Fed unwinds investment portfolio

The first month is in the books for the Federal Reserve's yearslong process of pulling the plug on its bond-purchase program, and markets took things in stride.

It's a massive undertaking for the Fed, whose investment portfolio swelled to nearly $4.5 trillion. Economists credit the program with helping push U.S. stock funds to returns of more than 300 percent since the spring of 2009.

A worry was that the program's end would mean pain for the stock market. But the stock market calmly set more records in October, as the Fed let $10 billion of Treasurys and mortgage-backed securities in its portfolio mature without reinvesting the cash. The pace will gradually rise to $50 billion per month by the end of next year.

The bond market was more exciting in October, as the yield on the 10-year Treasury touched its highest level since March.

A lot is riding on whether markets can remain calm as the program further winds down. Studies have suggested it was powerful enough to pull the yield on the 10-year Treasury note down by 1 percentage point as of the end of last year, when it was around 2.47 percent.

Most analysts expect a gradual increase in the 10-year yield from the current 2.35 percent, as the Fed continues with the portfolio unwinding and raises interest rates. But if rates rise faster than expected, markets could get upset.

A jump in rates would suddenly rob stocks of some of their attractiveness. Even though stocks look expensive relative to history based on how much profits companies are producing, they don't look expensive when compared against the small interest payments from bonds. If rates jump, investors would likely be less willing to continue paying such high prices for stocks relative to earnings.

Other central banks are still pumping lots of stimulus into the global economy. The European Central Bank is still buying 60 billion euros of bonds each month, for example. The Bank of Japan, meanwhile, is also buying corporate bonds, real estate investment trusts and exchange-traded funds.

So, what could upset the market? With the economy already healthy, additional stimulus from the U.S. government through tax cuts or other forms could heat up the economy even more and cause prices to jump. If that were to happen, the Fed may find itself forced to raise interest rates more quickly than expected.