The relevant provision of the Constitution states: "[F]or any Speech or Debate in either House, [Senators and Representatives] shall not be questioned in any other Place." (Article I, Section 6). I do not see how this creates an exemption from an otherwise lawful search.
Congress has been out of control--usurping Executive authorities and poking their noses into everything--since Watergate. Bush has been doing a fair job of correcting such abuses. The only branch of government more messed up is the Judicial.