WHAT is the appeal of heavy metal? “Hanging out with guys and doing dumb s – – t,” says one practitioner. “It’s just that simple.”

That fist-pumping spirit animates the hilarious and insightful documentary “Metal: A Headbanger’s Journey.” It’ll make you want to dig out your Whitesnake T-shirt. It might even convince Tipper Gore that heavy metal thunder is all in good fun.

Narrated by Sam Dunn, a Canadian anthropologist and metalhead, the film out-taps “Spinal Tap” in funny interviews with Alice Cooper, Dee Snider and Black Sabbath guitarist Tony Iommi, who wonders why the church disapproves of his band. “We used to use crosses onstage on fire and stuff like that,” he muses. “Probably didn’t help.” The notion that metal was born in opera houses is surprisingly credible; Richard Wagner wrote music for the octobass, a seemingly Derek Smalls-designed instrument so huge that the fingering had to be done by a guy on a ladder. Rafters shook. “If Richard Wagner was around today, he’d probably be in Deep Purple,” says one observer.

There is also an amusing look at more recent history – Twisted Sister’s Snider and a not-yet-psychotic Al Gore once faced off in Senate hearings because of Tipper’s batty crusade to censor rock music.

The film doesn’t neglect the genuinely scary aspects of metal: a Norwegian band actually burned down churches. But most fans seem as harmless as Dunn, whose love for the material is touching: “If metal doesn’t give you that overwhelming surge of power and make the hair stand up on the back of your neck, you might never get it.”

Even the end-credits list of interviewees is a giggle. Let’s play Who’s Got the Best Metal Name Ever: Gruttle Kjellson? Necrobutcher? Or maybe George “Corpsegrinder” Fisher? And who is this guy Chuck Klostërmann?

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METAL: A HEADBANGER’S JOURNEY

[*** 1/2] (Three and one-half stars)

Rocks you like a hurricane.

Running time: 96 minutes. Rated R (profanity, grotesque images, nudity, sexual content). At the Sunshine, Houston Street, near First Avenue.