I had my old towers on my E36 M3 crack eventually using Bilstein coil overs. First they rounded then cracked.
What car do you drive?
I welded the crack and fitted strut mounts from the four door M3 which comes with a special rubber spacer at the top that absorbs a lot of the impact. It seemed to help as I never got a crack again.
Take the car to Ted as Kobus adviced. Possibly there might be a rebound setting wrong or something.