One possible cause of the squeak is if the top hat (top coil seat) is not located properly on the bush or washer. This will allow it to rub against the shaft and cause a terrible squeaking noise. If left unchecked, it will eventually cut into the top hat and/or shaft. You will need to pull the strut apart to check this.
Here's a photo from seb's thread showing what happened to his top hat after it was rubbing on the shaft.
Another possible cause of the squeak is if the coil is rubbing against the bottom coil seat. There needs to be a small gap to allow the coil to move. If there isn't, it will continually to rub against the seat and cause a similar squeaking noise. Once again, you would have to pull the strut apart to fix this problem. It’s not the best picture, but you can kind of see the gap between the coil and the step on the seat.