It seems there were a sound of boom heared over Chengdu a few hours ago (about 11:00 Beijing time), cause is unclear. However, there was a text message sent to phones in Chengdu, stating all Chinese citizens have an obligation to keep secrets. Some local news report that the emergency management agency of Chengdu responded and confirmed that the boom is "related to aircraft test flights".
Update:
Across all social media platforms, there are many posts with #成都巨响# (Chengdu big boom), some also mention there were actually two booms, another one was at about 2 pm Beijing time.
Chengdu is rainy today so it might also be just a false alarm, could be just a lightning.