Thanks for all the replies.
I would reward her for being quiet around the machine, but that never happens. She doesn't seem scared but does get hyperactive. Her tails wags like crazy and she chases the sweeper (it's a cylinder type), all the time barking and pouncing at it. She'll even bit it a few times. She's much the same with the lawnmower but resorts to trying to bite the cable instead. With the mower she barks at it even when it's not switched on, whereas the vacuum cleaner is left alone when not switched on. The hairdrier gets the same treatment.
The one thing that all items have in common is a high pitched whining sound from the motor so I'm sure that's what's bothering her. We tend to keep her in a different room when possible.
Our vacuum is an old Hoover (8 years old?) and I've read that most manufacturers make quieter models although I can't find any actual noise level figures. Miele is the front runner from my research - reliable, sturdy, powerful and quiet setting available. Dyson seem to have a mixed review, many people love them, many people return them or buy another brand and leave the Dyson in the cupboard. I got access to Which? reviews and Miele had 80% of the top 20 rated machines. Dyson didn't make it into the Top 20.
Thanks again for the help and input. Time to suck it and see, so to speak. Miele it is. I think.