What was the strange noise I heard?

Hi, I was at Fowlmere this evening hoping to see some activity at the barn owl nest box. They were peeking out but no more than that, and after a while I decided I needed to leave to walk round the trail and back to the car before it got completely dark. On the path back towards the car park, I heard something making a lot of noise. It sounded almost pig-like and was quite loud. I couldn't work out at all what it might be and hope someone might be able to tell me. Could it have been frogs or toads? A warthog?! Hippopotamus?! It would be great to know!

Thanks, Amanda