Football racism still rife in 2018 World Cup host Russia
The noise was hard to distinguish at first, mixed in with the fans' cheers and singing.
But once spotted it was unmistakable: every time a black player from either side got the ball, a section of supporters behind each goal broke into monkey chants.
At one point a laughing fan performed an ape dance to match his grunting.
This was a football match in Russia, the next host of the World Cup.
Officials here play down suggestions that racism is a serious problem in the Russian game and say they are working to kick it out for good.
But those scenes at the match between CSKA Moscow and Zenit Saint Petersburg, two of Russia's biggest clubs, suggest that their efforts so far have had limited impact.
They followed soon after CSKA had been forced to play to two matches to empty stadiums, punished by Uefa for fan violence and racist conduct at a game in Rome.
"There is always something like this, coming from our opponents' fans," Zenit's Brazilian striker, Hulk, complained after the match in Moscow.
He ascribed the abuse to "ignorance and a lack of culture".
"If something like this happens during the World Cup, it will be a big problem," Hulk added.
Hulk was the first black player Zenit had ever signed. His arrival in Russia's second city in 2012 sparked protests by fans who described the lack of black players until then as an "important tradition" and an issue of "identity".
Last year, an official from football's governing body Fifa described the level of racism in the Russian game as "completely unacceptable" for a World Cup host country.
Piara Power, of Fifa's Anti-Discrimination Task Force, called on President Vladimir Putin to send a message that such attitudes would no longer be tolerated.
'Not very serious'
Hosting the World Cup is a matter of prestige for Moscow, especially as it faces increasing political isolation for supporting pro-Russian insurgents in eastern Ukraine.
There were already mumblings about a possible boycott over that crisis.
Then Western economic sanctions, piled on top of a plunging oil price, forced the government to cut spending on the tournament: two stadiums will be 10,000 seats smaller than planned.
And now there are questions over the treatment of black players and fans.
Sports officials no longer deny there is a problem but they do diminish it.
"There are problems everywhere but in Russia it's not very serious," Sports Minister Vitaly Mutko told the BBC.
"Sometimes fans shout racist things but we take measures. There are punishments. I don't think we stand out on this issue. There are lots of black players here and I don't see any problem."
Some of those players though, would disagree.
Earlier this season, Dynamo Moscow defender Christopher Samba suffered racial abuse from FC Torpedo fans. When he responded with what the Russian Football Union (RFU) called an "unpleasant gesture", he was given a two-match ban.
In another incident, a coach in Rostov - one of the 2018 World Cup host cities - commented that he had had "enough of the things" when asked if he was interested in signing a defender from Cameroon. He was referring to black players.
The coach, Igor Gamula, also joked that his squad might have Ebola, after several footballers fell sick.
His comments earned him a five-match ban when they hit the international press. The coach apologised, but still insisted his "joke" had been misunderstood.
Gamula's attitude chimes with that of many fans who see their monkey chants from the stands as simply part of the "banter". The crowd shouts at players from the opposing side, the argument goes, and they will pick on anything.
But such behaviour has been illegal for over a year now.
The "Fans' Law" passed in December 2013 seeks to regulate supporters' conduct and introduced strict sanctions for violations including inciting hatred, violence and racism.
The penalties range up to a seven-year match ban, stiff fines and 15 days' detention.
A RFU official insisted the law was being implemented but could not provide statistics.
Torpedo Moscow had to play a match behind closed doors after the incident with Christopher Samba but at the CSKA v Zenit match we attended, neither stewards nor police batted an eyelid at the repeated racist chanting.
A CSKA official later told the BBC most clubs lacked sophisticated surveillance equipment to tackle the problem properly and prosecute.
"The authorities have only recently started to fight racism," points out Alexander Verkhovsky, from the Russian anti-racism group SOVA.
"Five years ago there was impunity, they didn't care. We do need more decisive action, but the fact they are doing anything at all is progress
He attributes the problem to the "lamentably high level" of racism in broader society.
"Football fans aren't good little boys," one CSKA supporter - Jan - commented during the match with Zenit.
"They're lads from the streets, and can have radical views.
"Racism does exist in Russian football. I don't think it will be eradicated for at least a decade."
But it is just three years now to the World Cup. Fifa wants the event to showcase zero tolerance for discrimination. Russia is still a long way off achieving that.