A 911 caller warned of further explosions right after the blast that rocked Manhattan’s Chelsea neighborhood and injured 29 people, The Post has learned. “I’m looking at the explosion down the block. There will be more,” the unidentified male said, claiming to be standing at 23rd Street and Seventh Avenue in the aftermath of the terrifying incident, according to law-enforcement sources Sunday. The bomb appears to have been a pressure-cooker device, sources said. Investigators have obtained surveillance video of a person placing another unexploded device — a pressure-cooker bomb — that was later removed from 27th Street, the sources added....