(CNN)Millennials are a bit of a mystery to Janet Yellen.
The head of the U.S. Federal Reserve said Tuesday that the behavior of millennials — which typically refers to a generation of people born in the 80s and 90s — has top economists scratching their heads.
"I think we're just beginning to understand how the millennials are behaving," Yellen said before the Senate Banking Committee. "They're certainly waiting longer to buy houses; to get married. They have a lot of student debt. They seem quite worried about housing as an investment. They've had a tough time in the job market."
As the economy continues to gain strength, Yellen said she expects more millennials to buy homes and start families. "But," she quipped, "we've yet to really see how this is going to affect that generation."
Tuesday kicked off Yellen's two-day back-to-back testimony on Capitol Hill this week. For the first time since becoming head of the U.S. central bank last February, Yellen is facing a Congress that's controlled by Republicans in both the House and the Senate.
It's not exactly Yellen's first rodeo -- the 68-year-old Fed chair has had plenty of practice being in the hot seat, having endured numerous hours-long congressional hearings and nationally televised Fed press conferences in her first year on the job.
But her Hill appearances come at a critical juncture for the economic recovery, and her answers to lawmakers' questions -- particularly those that address the issue of when the Fed plans to hike interest rates -- are coming under heightened scrutiny.
Yellen offered an upbeat assessment of of the economy to senators on Tuesday. While she didn't comment specifically on when the Fed is likely to raise the federal funds rate, she suggested that a change in guidance would be coming soon.
Another popular topic that came up several times on Tuesday was the so-called "Audit the Fed" bill. An idea that's gained real momentum in recent years, the proposal would require the Government Accountability Office to conduct a full audit of the central bank's monetary policy deliberations. It's become a despised topic among top Fed officials who have spoken out forcefully against the idea.
Yellen made her views on the topic once again crystal clear on Tuesday: "I strongly oppose 'Audit the Fed,'" she told lawmakers, warning that the proposal would place "political pressure" on the central bank.
Yellen's testimonies this week come as congressional criticism of the Fed -- particularly from Republican lawmakers -- has noticeably intensified in recent years. With Republicans now setting the agenda in both chambers of Congress, Fed watchers anticipated plenty of sparks to fly.
"Recent criticisms of the Fed have come mainly from Republicans," said Alan Blinder, former vice chairman of the Federal Reserve board. "With a Republican majority on the Senate Banking Committee, and an even bigger majority in the House, it seems a reasonable guess that Chair Yellen might face some tough questioning."
Things won't get any easier for Yellen on Wednesday, when she faces a much larger -- and typically more combative -- group of lawmakers in the House Financial Services Committee.
Republican members on the House panel are preparing a long list of grievances to air with Yellen, including concerns about the 2010 Dodd-Frank Wall Street reform law's negative impact on community banks and small businesses, and an uneasiness about whether the Fed has become too powerful since the financial crisis.
Rep. Bill Huizenga, the chairman of the monetary policy subcommittee who has proposed that the Fed be required to use a rules-based approach for setting policy, said in a statement to CNN that he's eager to discuss "measures that will increase transparency at the Federal Reserve and make the Fed more accountable to taxpayers."