When a male gorilla let out a low, two-note murmur, the rangers responded by making the same sound, as if to say: "We've come as friends."
A young ape sat high in a tree overhead, yanking down a feast of leaves, while a second was splayed out in the tall grass, slapping his head and kicking his legs playfully. But it was clear the male, an enormous animal with glistening white teeth, was keeping an eye on things.
Virunga National Park, a World Heritage site in the restive eastern region of the Democratic Republic of Congo (DRC), just might be the world's most spectacular, under-visited park.
Stretching 185 miles (300 km) north to south and encompassing 2 million acres (81,000 hectares) of lush forest and savannah, it is Africa's oldest national park, established in 1925 when Congo was still a Belgian colony.
It is also the only park in the world with three kinds of great ape - mountain gorillas, chimpanzees and lowland gorillas - and is home to many of the world's mountain gorillas, which are thought to number fewer than 800.
But Virunga, the setting for American conservationist Dian Fossey's 1984 book "Gorillas in the Mist," is difficult to get to and the security situation is perennially uncertain.
Two decades of conflict violence, hunger and disease have claimed the lives of millions of people in this central African country, once known as Zaire.
The park was closed throughout last year, as rebel groups, including the M23, used it for cover during an uprising. Virunga reopened to visitors last January.
Just days after my visit in April, Virunga's chief warden, Emmanuel de Merode, a Belgian prince, was ambushed and seriously wounded.
"The vast majority of visitors wouldn't come to Virunga because of their perceptions of safety," de Merode told me days before that incident. "But we're not ready to take on vast numbers of visitors, so we gladly accept those who are prepared for, or even welcome, a level of risk."
EMERGING FROM FOG OF WAR
Virunga is a day trip from Goma, which sits on the banks of Lake Kivu in the shadow of the Nyiragongo Volcano, and has a number of hotels and restaurants. Visitors can also stay in the park at Mikeno Lodge, which is right in the forest. A gorilla trekking permit costs $465 per day.
My trip was supported by the International Women's Media Foundation as part of a reporting fellowship in Congo's North Kivu province.
The journey from Goma to Virunga takes about two hours, but on Congo's dilapidated and unpaved roads it was a bone-rattling drive that only got worse on the last leg, when bumpy turned to downright rocky.
Two rangers met my group and set out the ground rules: stay quiet and wear surgical masks to protect the apes from human germs. One of the rangers picked up a gun and off we went.
The largest living primates, mountain gorillas were habituated to human contact some three decades ago, and from 1988 till 1993 up to 10,000 visitors came to Virunga every year.
But following the Rwandan genocide of 1994, violence crossed over into DRC and the region has moved from one conflict to another. More than 5 million people have died from war and an ensuing humanitarian crisis in DRC since 1998, making it the world's deadliest conflict since World War Two, according to the International Rescue Committee.
The animals also face threats from poachers, squatters and charcoal burners who destroy their forest habitat. Virunga's Gorilla Sector suffered repeated attacks in 2007 during which 10 mountain gorillas were killed.
The rangers who patrol the park have also paid a heavy price. Since 1996, 140 rangers have been killed.
Through it all, even the most intrepid travelers have stayed away and Goma has become synonymous with war and suffering.
Conservationists now worry about oil exploration. British oil company Soco International has begun conducting seismic tests in the region, over the objections of conservation groups.
JUST A FEW FEET AWAY
After three hours of hiking through dense forest, I almost slipped in a pile of gorilla dung. I then knew we were close.
A light rain began to fall and we could see flattened patches of bush where the animals had passed. I looked nervously right and left, half expecting an ape to step into my path.
We walked for about 20 minutes more. The rangers gestured that we should arrange our masks.
And suddenly, there they were. The gorillas were nearly camouflaged, sitting under a canopy of leaves, and it took some time to register their presence.
A ranger sounded notes of assurance and gestured for us to follow him to a clearing. And there, we found ourselves mere feet from a male gorilla three times our size, with nothing to shield us save for the musical murmuring of our guide.
I was reminded of Fossey's description of her first encounter with the gorillas of Virunga.
"Peeking through the vegetation, we could distinguish an equally curious phalanx of black, leather-countenanced, furry-headed primates peering back at us," Fossey wrote. "Immediately I was struck by the physical magnificence of the huge jet-black bodies blended against the green palette wash of the thick forest foliage."
Tourism here has been on the upswing since de Merode took over as park director in 2008.
Some 550 visitors came to the park in 2009, swelling to 3,300 by 2011, he told me. In 2012, the park was on track to see 6,000 visitors, but then the war broke out in April. This year, de Merode said he expected about 1,000 tourists.
But he said this period of relative peace cannot be taken for granted and he has championed several economic development projects.
"We've just come through one of those terrible periods of horrific violence and we have to act very quickly," he said.