Cat yowling at invisible threat in the evenings

[Wotsupmog]

Our 8 year old male cat Tristan, who is normally very chilled, suddenly gets stressed most nights around 10pm. He prowls around the front room in our Victorian house making a very loud, short cry and stretching his mouth wide, particularly concentrating on two corners of the outside wall which are close to drains (could be a coincidence, but I'm thinking rats in drains, though he doesn't seem scared of rodents at other times). It goes on for about 20 minutes then stops and he relaxes.
Has anyone seen anything like this or find a way to sooth a stressed cat who may be trying to protect the home from predators that we can't hear or smell?

Fox outside? I think they tend to stick to the same route every night

If you go outside that would probably be enough to see them off and let T return to normal

We wondered why our cats were breaking into the hundreds of cupboards we have in the upstairs bedroom of our former bungalow until we discovered there were bats in the roof space and this was what they could hear.

Ghost?

Fox, or another cat outside? My cat did this, wandering around protecting the house at night and making a racket like a seagull. I think you just have to let them get on with it, short of putting deterrents outside. I moved to a high rise flat recently and really don’t miss her guarding noises at night.

Thanks, all interesting thoughts. Tristan seems to target skirting board level so I fear something in the wall, but it might be coming from outside. He ignores the hedgehog but a clever fox is possible though we never see one.
Good thought about the ghost but do they keep such regular hours? It always happens around 10pm.

Fascinating! Breaking into cupboards is part of T's evening routine but he never finds anything. Ours is a 3 storey house and he only does it downstairs so not bats here. But the animal must be nocturnal.