The stock market staged an epic rally after Donald Trump was elected president of the United States. It was one of the best post-election performances in history.

But now that Trump is about to be inaugurated as the 45th commander-in-chief, the enthusiasm has died down a bit. Stocks have been relatively flat so far this year.

Yes, the Dow, S&P 500 and Nasdaq are still not far from their all-time highs. So it's not as if Wall Street is panicking about Trump.

However, some experts think that the rally won't resume unless (or until) Trump gives more clarity about his economic plans -- especially his stimulus proposals.

"It still remains to be seen whether or not these pro-growth policies will be implemented and, if they are, how much time will elapse before they have an effect on the economy," said William Lynch, director of investments for Hinsdale Associates, in a report.

"Until this becomes clearer, stocks are likely to trade sideways," he added.

Trump's frequent attacks on blue chip companies (both on Twitter and traditional media) may not be helping either.

Related: The biggest winners and losers from Trumponomics

He's criticized Boeing (BA), Lockheed Martin (LMT), United Technologies (UTX), GM (GM) and Ford (F) -- as well as foreign car companies Toyota (TM) and BMW (BMWYY) -- and threatened big tariffs on companies building their products in Mexico instead of the U.S.

And in Trump's first press conference since the election last week, Trump spent a fair amount of time blasting the pharmaceutical and biotech industries for high drug prices.

Lynch wrote that investors would prefer to see "details on his policies of cutting corporate taxes, deregulation and fiscal stimulus" instead of more broadsides against big businesses.

Melissa Brown, senior director of applied research for risk management software firm Axioma, agreed that investors are still trying to figure out what to make of Trump's frequent tweets. Trump's unpredictability has elevated the amount of risk.

"The reward for being right is more than usual now, but the penalty for being wrong is more than usual too," she said.

Brown added that trading volume on Wall Street has been extremely low since the election, a sign that investors are sitting tight and waiting for more concrete economic plans from Trump.

"Low volume has been a weight holding back some enthusiasm," she said. "Investors are not panicking, but not running out to buy either."

Related: Trump trade talk sends dollar lower

Volume could pick up soon though. The transition period is about to end and Trump will get down to business.

"As early as next Monday the new administration will get to work, and with it comes the possibility of an increase in market volatility. What were heretofore opinions will now carry the weight of policy prescriptions," said Ameriprise chief market strategist David Joy in a report to clients.

But David Winters, an activist investor who runs Wintergreen Advisers, urges investors to avoid the noise. He says that as long as Trump is successful in cutting corporate taxes, many big U.S. companies will thrive.

Winters owns cigarette giant Reynolds American (RAI) -- which just agreed to sell to British American Tobacco (BTI) -- railroad Union Pacific (UNP) and Google parent Alphabet (GOOGL) as top holdings.

"We're positioned well for a Donald Trump world. Most of the portfolio should benefit from his pro-business policies." he said.