Your dog used to lie there while you cleaned around them. Now the cupboard door opens and they are gone, upstairs, behind the bed, panting.
Why a familiar noise turns frightening
New wariness of ordinary things is common in the second half of the first year. Owners often meet it as a bold dog turning cautious inside 2 weeks. The vacuum is a frequent first one, because it is loud, it moves, and it chases them around the room.
Nothing has gone wrong in your dog's upbringing, and nothing about this means your dog is fragile. It is not misbehaviour, and treating it as though it were will cost you weeks.
Noise sensitivities also tend to spread if they get rehearsed. A dog who runs from the vacuum every week can start reacting to the hairdryer, the blender and the smoke alarm as well.
Safety note
Book a veterinary examination if this appeared suddenly. Sudden behaviour change warrants one, and pain frequently presents as a change in behaviour rather than as obvious lameness. Get help now if your dog panics rather than avoids, cannot eat when the house is quiet, or hurts themselves trying to escape. The same applies if most household sounds now frighten them. Severe noise fear needs a veterinarian and a qualified behaviour professional, and there is treatment for it.
This week: let your dog leave
The first job is to stop the rehearsal. Working below the point at which your dog reacts is the condition for learning near something frightening. A dog in the room with a running vacuum is nowhere near it.
Put your dog in the furthest room with the door shut and a chew before the vacuum comes out. Every time, without exception.
Turn a radio or a fan on in that room. A dull background sound takes the edge off the noise coming through the wall.
Vacuum a small area, then stop. Ten minutes of cleaning with your dog settled is worth more than a whole house with your dog hiding.
Never carry your dog toward the vacuum to show them it is harmless. That is the fastest way to make next week worse.
Then 3 minutes a day, in the right order
Stand the vacuum in the middle of the room, switched off, and scatter food around it. Do this for 2 days and nothing else.
With your dog 20 feet away or in the doorway, move the vacuum 2 feet with the motor off. Feed while it moves. Stop feeding when it stops.
Switch it on in another room with the door closed, for 3 seconds, while you feed your dog where they are. Then off, and stop feeding.
Build the sound closer or longer, one change at a time, only once your dog can eat easily at the current step.
If your dog stops taking food, you have gone too far. Go back 2 steps and finish there, not on the hard rep.
That order matters more than the speed. Relaxation is trained as a behaviour in its own right, in short repeatable sets below your dog's threshold. This is that principle applied to a noise.
Two things that set it back
Correcting the barking or the hiding. Punishment-based approaches are associated with fear and avoidance, and here the fear is the whole problem.
Flooding your dog with the noise until they stop reacting. A dog who has gone still and quiet is often not calm, and the next session will tell you so.
Comforting a frightened dog is not one of these mistakes. If your dog comes to you, they are welcome. You cannot make fear worse by being kind to a scared dog.
Behaviour change is built from management plus reinforcement together, and reward-based methods are what the veterinary behaviour guidance recommends for fear work. Progress here is usually weeks, and it moves around from day to day. Nobody can promise you the dog who slept through the cleaning at 8 months old.
Today's version, in one line: stand the vacuum in the room switched off, scatter 10 pieces of food around it, and put it away.