I am not a strut expert... But it sounds like to me that the spring
may be on wrong, or the strut isn't installed correctly. Did you do
only one side? If so, compare the existing side with the new side and
see if there is a difference.

If I understand your description of the problem:  The top of the strut
should not be moving around where it is bolted up to the chasis.  If it is,
tighten it, there should be a torque specification for that nut.  Another
problem could be that the top of the spring can be moving around in it's
cradle.  Sometimes there's a plastic  washer that fits between the spring
and the cradle that allows small amounts of movement between the two .. if
the washer is worn out or missing you can hear the spring snap as it moves
with respect to the cradle.  Check for wear and replace.  Also check to see
that the top of the spring and cradle are aligned properly, they are shaped
to fit a particular way.