A rock-wielding menace bashed a straphanger in the head during a random attack inside a Manhattan subway station Wednesday afternoon, according to police and sources. The 56-year-old man was walking inside 14th Street and Eighth Avenue station in Chelsea on the mezzanine level when a brute came out of nowhere and whacked him multiple times in the head with a rock around 12:30 p.m., cops said.“He took a good shot to the head,” a construction worker who witnessed the unhinged assault told The Post. The attacker then ran out the station, according to police and the witness. The injured victim, who suffered lacerations to his head, was taken by medics to NYC Health + Hospitals/Bellevue, where he was listed in stable condition. Police said the perp was still on the loose Wednesday afternoon.The underground mayhem came hours after a 54-year-old man was stabbed to death on a platform inside the Coney Island-Stillwell Avenue subway station Tuesday night around 11:20 p.m., cops said.Moments before the bloodshed, the victim fought with his attacker on board a southbound N train, according to sources.His assailant had not been captured as of Wednesday afternoon. Additional reporting by Joe Marino and Amanda Woods...