At first it sounded like someone was trying to start an outboard motor. Or, seeing we were in the middle of a forest, maybe it was a dirt bike. Or was it a chainsaw?
It started with a deep, rumbling growl that rose steadily in volume and menace before descending into a series of guttural grunts.
Then I clicked: It had to be a wild pig. We were about to charged by a heavily tusked, hiker-hating boar. But why was the sound coming from a tree?
Eventually my guide pointed out what appeared to be a grey, furry basketball lodged high in the fork of a gum tree. The mystery sound was in fact the mating call of an amorous koala. They might look cute and cuddly but when these randy marsupials are looking for love they sound anything but.
Their mating calls were so frequent they became the soundtrack to my two-day taster of the Great Ocean Walk. My guide thought maybe the return of cool, dry weather after heavy rain had awakened their libidos.
The Great Ocean Walk starts at Port Apollo on the Victorian coast, about three hours' drive west of Melbourne where the famed Great Ocean Road veers inland to avoid the Otway Range, and ends 100km later at the 12 Apostles, an iconic series of rock stacks carved by the sea.
Officially the walk takes eight days but can easily be done in six. I could spare only two days but guide Marie Killeen picked the two best bits for me.
A walking holiday around the Lucky Country is a very different experience from tramping in New Zealand. Each morning a driver drops you at the starting point with a map, instructions and packed lunch, then picks you up in the afternoon further down the trail to ferry you to that night's lodgings.
The beauty of it is that you only have to carry a small daypack, plus you get to eat magnificently and sleep in comfortable beds.
It works well in Australia because many of their long-distance trails have road access at regular intervals, unlike multi-day tramps in New Zealand.
Marie and I started at Marengo, a few kilometres beyond the official start at Apollo Bay.
For the first half day we were never out of sight of the Southern Ocean as the track led over headlands, across beaches and along clifftops.
At Elliot Stream the trail swung inland — that's where we heard our first lovesick koala — and through pretty much every type of bush you'll find on the Victorian coast.
There were aptly named stringybarks, lush gullies dominated by treeferns, manuka-like thickets of paperbark, and forests of towering mountain ash which Marie, a walking botanical encyclopedia, told me is the world's tallest flowering hardwood.
Our first day's walk ended at Blanket Bay where our ride was waiting. In 20km we'd seen just two other walkers.
"That's the beauty thing about this track. You don't see a lot of people," Marie said.
We stayed at Aire Valley Guesthouse, a historic farmhouse converted into a B&B and run by the ebullient Susie McDonald, an outstanding cook, winemaker, Francophile, art lover and Chinese medicine practitioner.
Our second day started at Milanesia Beach, a wild and deserted stretch of sand, before climbing a series of stairs to Bowker Pt and a cliff-hugging track offering superb views of rugged headlands battered by the sea.
After lunch we hit a stretch of trail Marie called the Roller Coaster, a strenuous few hours of up and down rewarded by a gently undulating track through shady eucalypt forest, emerging at a viewpoint called The Gables.
There were fewer koalas but we did see wallabies, soaring wedge-tailed eagles and a blue-tongued lizard chilling out on the track. But no catalogue of Australian wildlife would be complete without mentioning the flies. You can almost hear them laugh as you try in vain to brush them off.
It's not hard to see why the Great Ocean Walk is one of Auswalk's most popular trips. And why Marie doesn't get bored despite her 50-plus traverses equating to 5000km.
"I see it in different weather and different seasons, and seeing it through the eyes of my customers is really lovely. Also, it's the surprise of the wildlife — the koalas peering down on you, the swamp wallabies, the echidnas, the occasional snake," she said.
For her customers, the appeal was the sheer variety of habitats and environments.
"You're on a cliff edge one moment, in a forest the next, then on a beach. You're in low coastal scrub one moment and among the world's tallest flowering hardwood the next."
We had only walked about a third of the trail but there was no way I was going to miss the scenic highlight, even if we had to drive.
The Great Ocean Walk ends at the 12 Apostles, a dramatic series of rock stacks carved by the sea. The brittle rock is constantly eroding and changing, which explains why only about eight Apostles remain.
I had grand plans to get up early and see the Apostles at first light. That way I would also beat the crowds which can hit 12,000 a day in peak season.
But when the dawn sky was grey and uninviting I lingered in bed instead and arrived at the coast mid-morning — just as the sun broke through a bank of cloud and cast a golden light on the Apostles rising from a dark and crashing sea. Spectacular.