Reporters said tyres were slashed, while cars were kicked and covered in flour and eggs.
Greater Manchester Police were called to the scene earlier and said inquiries were continuing.
Giggs was named in Parliament as the married footballer exposed on Twitter as having an injunction over an alleged affair with reality TV star Imogen Thomas.
Ferguson banning order The 37-year-old midfielder is United's most senior player, having appeared in 613 games for the club, since 1990.
Giggs was absent from an earlier open training session in front of the world's media, as part of United's preparations for the Champions League final against Barcelona at Wembley on Saturday.
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Associated Press (AP) reporter Rob Harris asked how important Giggs was for Saturday's Champions League final against Barcelona.
Sir Alex replied: "All of the players are important, every one of them." He then moved on to another question.
He was later overhead on a microphone telling a press officer to "ban him on Friday". Another press conference is scheduled the day before the Champions League final.
United spokesman pointed out that as Friday's event is organised by Uefa, the Old Trafford club has no jurisdiction to ban Harris.
Lib Dem MP John Hemming named Giggs during an urgent Commons question on privacy orders on Monday.
Using parliamentary privilege to break the court order, he said it would not be practical to imprison the 75,000 Twitter users who had named the player.
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