It’s all there in the first minute: there’s a low-mid whoosh that’s clearly guitar feedback, like a Jimi Hendrix power chord trailing off; there’s a bit of an electric rattle, perhaps a fluttering speaker cone gasping for air; then come the high-pitched screeches, perhaps bringing to mind a grainy video image of seagulls circling over an open sea filled with radioactive garbage. From there, a ringing squall is folded in, an unstable mess of harmonics that shudders and quakes like nerve impulses curling down a human spine. And with that, we’re deep into Lou Reed’s Metal Machine Music.  For a good 15 years after its 1975 release, Metal Machine Music, a double album of avant-garde noise put out by a rock legend who was only starting to get his commercial due, was discussed as a gesture more than as music. Explanations for its existence proliferated.