City will have been content with the first knockout draw after Copenhagen needed to win just two of their Group A games to progress behind winners Bayern Munich.
One of those came against United in the Danish capital, fighting back from 2-0 and 3-2 down to triumph 4-3 to send the 38,000 fans in attendance into raptures.
City started the defence of their maiden Champions League crown by cruising through Group G, winning all six games to finish ahead of RB Leipzig, BSC Young Boys and Red Star Belgrade.
And as they kicked off their knockout run in Copenhagen, those watching noticed they were hearing the same thing from their TV sets.
Sektion 12, the towering stand at Parken that houses the club’s fierce ultras, fuels the stadium’s raucous atmosphere, especially on European nights.
Among the relentless chanting, clapping and waving of flags, viewers quickly picked up on what sounded like a loud siren blaring out during the action between City and the hosts.
Many viewers were convinced they were hearing things, while others thought the siren noise was coming from outside their windows.
The Champions League, Europa League and Conference League never fail to showcase a range of fan cultures and stadium atmospheres across different countries.
And Copenhagen, Denmark’s most successful club, pride themselves on having one of the loudest receptions in the continent, which proves challenging for any visitor.
United also experienced that first-hand in the group stage, when the siren and a megaphone were in use throughout the game and as Bruno Fernandes took a second-half penalty in front of the ultras.
After the Red Devils skipper succeeded from the spot, he aimed his celebration at Sektion 12, while Alejandro Garnacho followed his lead by making a shushing gesture.
Before Tuesday’s clash, City had failed to leave Parken with a victory in their two previous trips, drawing 2-2 in the UEFA Cup in 2009 and 0-0 in last season’s Champions League campaign.
But Guardiola’s side entered half-time on Tuesday evening with a 2-1 advantage through Kevin De Bruyne’s 10th-minute opener and Bernardo Silva’s second go-ahead goal on the brink of the break.
Magnus Mattsson drew Copenhagen level in between on his club and Champions League debut, profiting from an Ederson mistake.
