Coil spring noises - dangerous?

When I make the suspension compress (by going over a bump or turning the vehicle), sometimes I can hear the coil spring suddenly shift. It's a metallic thump.

It happens more often in colder weather, but it doesn't happen often enough to make it happen on demand for the dealer.

Dealer inspected the suspension and says everything is fine. I've never had a car do this, and it's only the left-front that does this. Any thoughts? Is this a sign that it is going to break? $1,300 to replace the front springs.

This is a Volvo XC70, 45k miles. Left-front spring.

Thanks.