No. Under official Monopoly tournament rules, you cannot hide your money: all of your cash has to stay above the table and visible to the other players.
The rule exists to stop players from secretly tucking bills under the board or on their person and then bringing them back into play later. Visible does not mean neatly organized, though. You are not required to split your cash into separate piles by denomination, so a single stack on the table is acceptable. The same tournament standards also say the Bank is the only entity that can lend money, which it does through mortgages, and that trades between players must involve tangible assets they already hold, such as cash, properties, and Get Out of Jail Free cards, never a straight cash-for-cash swap. The guide was written by Tim Vandenberg, runner-up in the 2009 U.S. Monopoly National Championship, and is published with Hasbro’s permission. It notes that families are free to use their own house rules at home, but the official rules are designed to make the game play fastest and most strategically.
Source: Tim Vandenberg, The Methods, Math, & Myths of MONOPOLY (Hasbro-licensed handout)
Image: Monopoly money, John Morgan, CC BY 2.0, via Wikimedia Commons.