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fears, irrational, in babies
I have heard of this before. I am no developmental psychologist; this is the product of my own observations and reflection upon the phenomenon.
There seems to be a parallel developmental relationship of anxieties in general to the development of separation anxiety, which appears some time after 6 months and has a peak of intensity around 8-9 months. There is, at least as I judge from my daily dealings in the office with young children, another "spurt" of separation fears and stranger anxiety around 15 months as well. To me, the ability to form anxieties is probably part of the normal process of learning to fear and avoid potentially harmful things or situations. This would include separation from mother, potentially hostile strangers, potentially harmful animals, loud noises, and many other things.
As for what to do about it, I suppose a mother could attempt to desensitize the fear by holding the baby and reassuring him while someone briefly turns on the vacuum at first in another room at some distance. At first, the vacuum is turned on only briefly, and as soon as Baby becomes upset, it is quickly turned off, and Baby is comforted. This little drill can be repeated, Mommy laughing and obviously happy and unafraid of the vacuum cleaner, while holding Baby. These sessions would happen over a period of several days.
Soon the baby ought to tolerate the noise better; then mother would advance to brief periods of vacuum on with her baby in the room. Mother's constant reassurance will teach him not to fear it.