Hi!
We call it 'rumblestrutting' as it is usually accompanied by shifting the weight between the back legs. It is very common mild dominance behaviour, exphibited by both genders although more often by boars than sows; neutered or not. Sows mostly either rumblestrut when establishing a group hierarchy, when coming into season or experiencing a hormone spikes.
You have to get used to the fact that guinea pigs do not just have squeaks but also a variety of rumbly, ground sounds for their communication, which is much more differentiated than you would expect! Communication includes pheromones, body language and situational context in addition to the sounds piggies make, which is the reason why any guide can only ever be incomplete. Their hearing range reaches higher than ours, too.
We hiumans usually need to take in body language and situational context in order to interpret many similar sounding rumbles, grunts and squeaks, especially at first.
But if it helps you, here is an audio guide that is mostly correct, unlike so many other online ones.
Guinea Pig Sounds
In your case, it is most likely that one of your pair is coming into season.
Sow Behaviour
Bonding: Illustrated Dominance Behaviours And Dynamics
When Sows Experience A Strong Season (videos)