Kirk Hammett is wailing away on guitar. James Hetfield is screaming into the microphone. And Robert Trujillo is ripping through a big bass line.

But you can’t hear any of it, since the players are using in-ear monitors/headphones — as opposed to any type of regular speakers — to carry on the musical conversation with each other. And all that’s left to be heard are Lars Ulrich’s drums thundering over the precise acoustic work of the large symphony orchestra that surrounds the four men.

It’s an odd and unfulfilling experience, like getting to listen to just one part of an important conversation.

Then, after about 40 minutes, the speakers finally kick in — loudly — and we get to feel the full force of this latest collaboration between Metallica and the San Francisco Symphony.

And it’s awesome to behold.

Of course, there are not many people around at this moment to experience it. The band and the symphony are performing in front of row after row of empty seats at this private rehearsal at the Cow Palace in Daly City on Labor Day.

But it will be a much different story on Sept. 6, when Metallica and the symphony perform a sold-out show on opening night of the new Chase Center in San Francisco. The show — which will be repeated on Sept. 8 basically as a fan-club-only event — marks the 20th anniversary of their first S&M collaboration in 1999 at the Berkeley Community Theatre. It also kicks off what will be a busy season of concerts at the arena.

The hype around this concert has been huge, given that the Berkeley shows — which were captured on the Grammy-winning “S&M” album — have become the stuff of legends over the years. Yet, fans shouldn’t expect the S&M² concerts to be copycats of the original gigs.

“It’s a new group of people. It’s a new building. Robert Trujillo wasn’t involved 20 years ago,” Ulrich says just prior to the rehearsal. “I like that (this production) extends a respectful nod to what it’s been in the past. But, at the same time, it’s an opportunity for new discoveries.

“And I think that is the Metallica spirit — of continuing this sort of quest forward rather than being tied down to the past.”

Pressed to remember the past, and think back to the first S&M gigs,” Ulrich is quick to bring up one name.

“You can’t talk about ’99 without bringing in the now late Michael Kamen,” Ulrich says of the legendary conductor-composer- orchestrator-arranger, who died in 2003. “It really was his brainchild.”

And whose brainchild was S&M²? Ulrich isn’t quite sure who came up with the idea first, only that it made perfect sense when he heard the pitch.

“If you look at all the things that get thrown around in our world, that is 110 percent a no-brainer,” he said. “The whole (concept) came together fairly effortlessly.

“Everybody realized that this was the right thing to do for the greater celebration of this new arena and San Francisco and the Bay Area.”

And with the concert only four days away, rehearsals are in high gear at the Cow Palace — which, it seems only fitting to point out, was the Warriors’ original Bay Area home — as the mighty men of Metallica and the symphony musicians try to work out the kinks in the S&M material during this rehearsal.

The energy level is incredibly high, fueled by the twin towers of enthusiasm — Ulrich, who can barely stay seated at times as he punishes his drum kit, and conductor Edwin Outwater. (S.F. Symphony music director Michael Tilson Thomas, who is conducting a portion of the S&M show, would show up later at the rehearsal.)

Outwater is upbeat as we talk during a rehearsal break, saying that he had long hoped there would be another S&M and that he was enjoying the work that goes into melding symphonic and rock sounds.

“There are no stressful challenges,” he says. “They are all fun challenges.”

One of the challenges, at this point, is for the musicians not to bump into each other as they crowd the stage.

And it’s quite a crowd.

Producer Greg Fidelman (who did the band’s most recent album, 2016’s “Hardwired … to Self-Destruct”) is up onstage with them. Famed photographer (and director of the excellent Ian Curtis/Joy Division film “Control”) Anton Corbijn is circling around taking pictures. And there’s a film crew, with a mic boom towering overhead, presumably capturing some of the action for the upcoming documentary “S&M²,” which opens in Bay Area theaters Oct. 9 (see Metallica.film for more information).

Despite all of that, and the setting and process being so different from what they are use to, the symphony musicians seem amazingly cool in the midst of the Metallica mania. That’s certainly true for violinist Jeremy Constant, who was the concert master at the ’99 shows.

“The sense of adventure that they had with their own music was really amazing,” he said. “The fact that it turned out to be such a success was really heartening. Talk about a collaborative art form — the number of moving parts is really phenomenal.”

One of those moving parts has been basically practicing since high school, in one way or another, to be a part of this gig.

“I actually started with this music. I played in a garage band and we covered Metallica,” says Scott Pingel, who is symphony’s principal bass player for these concerts. “I joined the symphony in 2004 and I was so bummed that I didn’t get to do the S&M concerts.”

Then there’s Ulrich, who admits that the biggest challenge of collaborating with the symphony is “not being completely (expletive) intimidated by all these incredible players and incredible musicians.”

And that’s coming from a co-founder of the biggest metal band in the land.

“We do what we do and some even accuse us of being semi-good at it. And that’s sort of our niche,” he says. “But I believe that any musician will always have a tremendous amount of respect and humility around other players and other people who excel at their particular niche or particular craft.

“I take it very seriously that I am, for better or worse, the anchor of the whole thing. And you just want to be on top of your game and really focused.”