A straphanger slashed a man who dared to ask him to turn down his blaring music on a Manhattan subway train this week, according to cops and police sources. 

The 37-year-old victim was on a 7 train approaching Grand Central minutes after midnight Monday when he confronted a man who was blasting music from a speaker and asked him to turn it down, the sources said. 

Instead of simply lowering the volume, the suspect whipped out a folding knife and snarled, “Do you have a problem?,” the sources said.

He then slashed the victim on the upper lip, cops said. 

The suspect got off the train at Grand Central — but the victim stayed on board until the next stop, Vernon Boulevard and Jackson Avenue in Queens, authorities said. 

From there, he called 911 and was taken to Weill-Cornell Medical Center with non-life-threatening injuries.