Task: Question Answering
Input (English): Passage: At the beginning of December 1910 Smoller, using the name Joe Levi, visited Exchange Buildings, a small cul-de-sac that backed onto the properties of Houndsditch. He rented No. 11 Exchange Buildings; a week later Svaars rented number 9 for a month, saying he needed it for storage. The gang were unable to rent number 10, which was directly behind their target, 119 Houndsditch, the jeweller's shop owned by Henry Samuel Harris. The safe in the jeweller's was reputed to contain between £20,000 and £30,000 worth of jewellery; Harris's son later stated the total was only around £7,000. Over the next two weeks the gang brought in various pieces of necessary equipment, including a 60-foot (18.25 m) length of India rubber gas hose, a cylinder of compressed gas and a selection of tools, including diamond-tipped drills.With the exception of Gardstein, the identities of the gang members present in Houndsditch on the night of 16 December 1910 have never been confirmed. Bernard Porter, writing in the Dictionary of National Biography, considers that Sokoloff and Peters were present and, in all likelihood, were two of those who shot the policemen who interrupted their burglary. Porter opines that Peter the Painter was probably not at the property that night, while the journalist J P Eddy suggests that Svaars was among those present. Donald Rumbelow, a former policeman who wrote a history of the events, considers that those present consisted of Gardstein, Smoller, Peters and Dubof, with a second group in case the work needed to continue into the following day, which included among their number Sokolow and Svaars. Rumbelow considers a third group on standby, staying at Hoffman's lodgings, to have comprised Hoffman, Rosen and Osip Federoff, an unemployed locksmith. Rumbelow also considers that present at the events—either as lookouts or in unknown capacities—were Peter the Painter and Nina Vassilleva.On 16 December, working from the small yard behind 11 Exchange Buildings, the gang began to break through the back wall of the shop; number 10 had been unoccupied since 12 December. At around 10:00 that evening, returning to his home at 120 Houndsditch, Max Weil heard curious noises coming from his neighbour's property. Outside his house Weil found Police Constable Piper on his beat and informed him of the noises. Piper checked at 118 and 121 Houndsditch, where he could hear the noise, which he thought was unusual enough to investigate further. At 11:00 he knocked at the door of 11 Exchange Buildings—the only property with a light on in the back. The door was opened in a furtive manner and Piper became suspicious immediately. So as not to rouse the man's concerns, Piper asked him "is the missus in?" The man answered in broken English that she was out, and the policeman said he would return later. 
Question: What did Piper think was unusual enough to investigate further?
Output (English): the noise.