Mel, at about the same age my Yasin went through a stage where he would freak out every time DH put a cap on and he also freaked at the vacuum and he wouldn't let one of my friends take her scarf off when she came visiting. He got over it eventually but to this day I have no idea why he decided he hated caps and vacuums. I asked the MCN about it and she said that it was a really normal developmental stage. Some babies hate glasses. some hate beards, some hate the car, some hate vacuums and some hate loud noises.
My DH has managed to scare my little ones on more than one occasion but it hasn't stopped him from building a fantastic relationship with them. He has a very differant kind of relationship with the boys than I do - he is much more physical and prone to jumping out at them from behind doorways and having screaming competitions. Even though I don't always agree with DHs style of parenting I try not to interfere because I think that it's up to him to choose his parenting style not me (although we agree on the big issues like discipline, schooling etc). Obviously it's working for him because both my sons drop whatever they're doing whenever he comes home to open the door and give him big hugs (Mum's just can't do bear-hugs like Dads can ) and try and engage him in a wrestling match. That said, if he jumps out from behind a door a little too enthusiasticly and causes tears it's up to him to comfort them.