08/11/2003 at 01:00 AM EDT
They're too loud," a customer complained to the manager of a Chicago Johnny Rockets restaurant of the giggling group behind him. "Can you get them to shut up?" Overhearing the complaint, offender Cameron Diaz
, munching on burgers with beau Justin Timberlake
, let out another laugh. Just try restraining these two: Since Timberlake, 22, started touring with Christina Aguilera
in June, Diaz, 30, has eagerly hit the road with him. Following his Florida gigs, the pair headed to Chicago, where on July 22 they were spotted at a downtown Diesel store and with two pals at the '50s-style diner. "They were all laughing and joking around with the staff," says a Rockets employee. "They were having a really good time."
That night Diaz—along with Timberlake's mother, Lynn Harless, 42—attended his wee-hours solo set at Chicago's House of Blues. Diaz stood boogying in a private box alongside actress pal Elisha Cuthbert (24
) and seemed to be "way into it," says talent buyer Michael Yerke, "cheering and going nuts." Likewise, Timberlake seems nuts about her. He has been singing her praises lately to his kin. Says a family source: "Justin says she's a lot of fun."