scared hen lays soft egg????

Chickens will get used to all sorts of noise, it's just initially it will scare them. You may get a few soft or odd shaped eggs as stress can cause that, but your best bet is to get the mower out and put it on in the background for a few minutes, away from them so they can hear and see it but not feel threatened by it. Do this a few times a day for a couple of days, gradually moving it closer and you'll soon find that it won't bother them. My compound is next door to a chap who uses disc cutters and grinding tools in his workshop and after the initial fright, they no longer take any notice of it.