I bought Ds2 (4.8) a miniature "Henry" vacuum cleaner. It is similar to the ones our cleaners use and to the one he sees in school. It is battery operated and makes a noise, albeit much quieter than a real one.
It has been an enormous success. He usually leaves the house when the cleaners arrive. Today, he stood in the hallway greeting them saying "Cleaners, look, I got vacuum cleaner, he's called Henry, I have to clean the carpets". Then every time they switched their vacuum on, he switched his on too (in another room). At the end, he touched their vacuum cleaner, having established it was not plugged in.
I am hoping this will speed up his ability to cope with vacuum cleaners.
The cleaners were lovely, one of them turns out to have a daughter with oversensitive hearing, and she has offered to bring me a toy hairdryer next week. But I think I'll buy my own as it has gone so well.
will post separately about hand-driers.
Perhaps if I'd bought him one when he was 2, he'd never have got scared of them. He isn't scared of hedgecutters or drills (but they don't move towards him).