The sound is unmistakable: what begins as a far-off purr becomes a roar of motorcycle engines, thousands of them, crossing Memorial Bridge into the District.
"Oh my God, it brings tears to your eyes", says Bobbie Janavice, from North Carolina. "It's amazing, and there's just so many."
The cavalcade is aptly called Rolling Thunder.
For 25 years, veterans, many of them from the Vietnam era, have ridden here from across the country, to take this special ride.
The final lap, from the Pentagon to the Lincoln Memorial.
"It's not just a bunch of people riding their bikes through D.C.", says Peter Langer, of Bethesda. "It symbolizes our freedom."
"Welcome home brothers, that's what I have to say", said Vietnam veteran Chuck Wichowski, as the parade of motorcycles roared by.
Wichowski, who served in the Army's 1st Infantry Division in 1968, says the ride stirs up memories... good and bad, long buried.
"It brings up a lot of emotions, a lot of memories", he says. "It's good to be home, good to be alive. Good to have my legs."
They are survivors, ruefully acknowledging the passage of time.
A war that ended, nearly four decades ago.
"My executive officer and his bombadier-navigator got shot down over the North", Dean Janavice recalls.
Janavice, who served in the Gulf of Tonkin in 1972 and 1973, holds his memories of missing comrades close--all these years later:
"As far as I know, they're still up there somewhere. No one ever accounted for them, so he's on the wall, both of them on the wall."
There is plenty of patriotism, and honor at this event.
And a thankful welcome home:
Vernon Smith, an aircraft mechanic at Da Nang Air Force Base, says he's seeing it, more and more.
"When we came back from Vietnam, we didn't never have the welcome back like it is now", he says.
Then there are those like Crystal Wortham, the daughter of a Vietnam veteran, here to honor her father.
"I know I need to come and be here, if nothing else for his spirit", she says.
Wortham says her father, Joseph Wortham, who served two tours in Vietnam, died last summer from cancer, due to agent orange.
"I just think about how many people went through what my father went through, and all the different POWs and MIAs that are still missing", Wortham adds.
Amid the roaring motorcycles along Constitution Avenue, stand two Marines, wearing dress blues, locked in a solemn salute.
Both men stand stiffly at attention for hours--a silent, disciplined tribute to the fallen, the missing, and those who are here.
"You travel up here and see all the people on the bridge, waving, waving flags. It's an excellent feeling", Smith says. "Makes you proud. Proud to be an American."