Hi, welcome to frog forum! Only males will call, so if he is croaking he's a male. Their call is a low rumbling sound, kinda like a stomach growl or a belch. Assuming they sound like that, my own ABF calls in the evenings, and your frog is around the minimum size for them to start croaking. Most people are still waiting to hear them at that size. They can grunt, but they usually wouldn't do this regularly at the same time. From what you've said so far, I'd heavily suspect male.