Noted at the Republican National Convention:

* SOME NEW ORLEANS RESIDENTS probably were shocked Monday to see what appeared to be President and Mrs. Reagan driving around the city in an old DeSoto convertible.On closer inspection, people found out it was not the first couple, but a radio station's morning team with Ron and Nancy masks doing a promotional tour through the Crescent City. The shock was so great, says Renie Hale, WQUE promotions director, that a cab rear-ended the back of another car.

* BROOKS PATTERSON, Michigan delegation chairman, knows when he faces a challenge. Actor Charlton Heston, who played Moses in Cecil B. DeMille's "Ten Commandments," charmed the delegation during a brief appearance Wednesday.

Patterson then tried to top that act Thursday. "Yesterday, I brought you `Moses.' Today, I bring you Abraham," Patterson told the delegates as he flung an arm around the state GOP chairman, Spencer Abraham.

* REP. LYNN MARTIN of Illinois, one of the sharpest wits in the Republican Party, had a quick response to questions about a 41-year-old on the GOP ticket - the youngest nominee for national office since Dwight Eisenhower tapped Richard Nixon, then 39, for the No. 2 spot in 1952. Said Martin: "The same people who are saying Dan Quayle is too young were saying Ronald Reagan was too old."