We all heard it and it turned out we were all thinking it, too: "Whatever that really is, it sounded like a fairly displeased Tyrannosaurus rex."
Luckily, we had Paul and indeed, every tour group needs a Paul — someone who knows everything about everything and can share this vast, often pointless knowledge with a captive audience not yet sick of it.
So, as we fumbled through some of the world's most dense jungle on the way to Guatemala's famed Tikal ruins, we learned we were not entirely wrong.
In a thick Brummie accent (Paul is a 33-year-old science teacher from England), he told us that the dinosaur-type yell was made by monkeys.
But (as Paul explained in his best "but wait, there's more" teacher voice), the territorial cry of these howler monkeys once so impressed director Steven Spielberg that he recorded it and used it as the cry of a T-Rex in the Jurassic Park films.
So we were right, who would have thought a little monkey several kilometres away could make such a fearsome, guttural noise? Hopefully, the girl monkeys were impressed.
We were here for ancient Mayan ruins rising out of the wilds of Central America and not necessarily for the "movie trivia" segment of a Birmingham pub quiz, but Paul was on a roll.
Tikal is not just another collection of old ruins where you drive up, take photos and leave with your thoughts of the place bleeding into memories of all the other temples and ruins you've seen along the way.
For my money, Tikal rivals Cambodia's Ta Prohm in size and density of the surrounding jungle and, as Paul explained, just like those temples (which were used in Tomb Raider), it's not only Tikal's noisy monkeys who've starred in the movies.
Surprise surprise, Paul is a Star Wars nut. Giddy with the excitement of a pleasantly nerdy teacher missing the blank canvases of his students' brains, he told us that back in 1976 George Lucas was scouting for locations for a little outer-space movie he was making.
In his head the good guys would have an Earth-like planet as their home base and the grey pyramids above the jungle canopy he found in Guatemala were a perfect fit. With instructions from Paul to look out for the rebel base at Tattooine the next time we watched the first Star Wars film, we set about taking in what it was we were seeing.
We'd driven an hour from the pretty lake town of Flores and had entered the sort of thick jungle that reminded me of growing up in Malaysia. Once on foot, this is what really sets Tikal apart.
There are several huge Mayan pyramid sites in Central America, but this is notable for being one of the very few where — to get to it — you have to walk through relatively untouched rainforest. In that jungle we saw monkeys, parrots, toucans and coatis, as well as a couple of coach-loads of overexcited Guatemalan school kids.
Occupying an area more than 20sq km, Tikal is a series of grey pyramids of varying sizes and gradients. The city, built several hundred years before Christ, housed more than 100,000 people at its peak and, like much of the Mayan empire, was mysteriously abandoned 1000 years ago.
Since being rediscovered in the mid-1800s the ruins have been surprisingly tastefully restored. We saw this first-hand when we came to a clearing with a 61m pyramid at its centre.
At first I thought: "There's no way I'm climbing that, look how steep it is." Then, one by one we attempted it, climbing to the summit almost like we were climbing a giant ladder. Getting down was not going to be as easy.
Sitting precariously on top we could see other pyramids rising above the jungle. Best of all, there was no town in the distance, no visible roads; not even the small clearings around each of the pyramids could be seen. As far as we could see, it was just lush green jungle with little specks of what man mysteriously built centuries ago. Sitting up there we could see how this other-worldly scene appealed to George Lucas.
It even left Paul speechless.
Tikal and Flores top five
Sitting on top Temple IV at Tikal — there are not many times in my life that I've been that high up and still been able to see jungle and only jungle for miles and miles.
• Tikal's Grand Plaza — a clearing entirely surrounded by Mayan pyramids that is truly one of Guatemala's grandest sites.
• Flores' hotels — an hour from Tikal, Flores is full of small hotels with hammock-dominated roof-tops ideal for early evening parties as the sun sets on the lake.
• Swimming in the lake at Flores — the town is on a small island in the middle of the lake (Lago de Peten Itza) and the water is deep enough to jump in and not even come close to the bottom. Beautiful temperature too.
• Shopping in Flores — this is definitely one of Guatemala's prettiest towns (if you can get past the machine gun-toting security guards) and along its cobblestoned streets are dozens of pastel-coloured shops selling the sort of tourist trinkets your family might actually like to receive.
IF YOU GO
Flight Centre offers Guatemala holidays. Phone: 0800 427 555.
Tim Roxborogh was assisted to Central America by Flight Centre.