The spring has a rubber piece that is between the spring and the frame fittings at the top of the strut. It's probably worn down. Jack it up and get a flash light and inspect it as much as possible and make sure the bolts are tight at the top of the strut base (inside the engine compartment). It also could be a rubber bushing that is bad in one of the rocker arm or caster that is worn down. Check that too.
I was having the same issue, but it was because I had those rubber wedges inbetween the springs to stiffen the springs a little. For some reason it made a popping sound. I replaced them with a the full round rubber wedge(the kind they use for stock cars) and the sound went away. I didn't even know it was causing the sound.
I just got my wheel bearings replaced, driver side a couple of weeks ago then the passenger side today. Man what a difference, rides like new. They were growling at me, I just wasn't sure it was wheel bearings because my 97 Sable only has 49K on it.
Anyhow, after the driver side bearing was changed I began to hear a popping (clunking) sound when I would turn at slow speeds. Like in a parking lot when I'm driving straight and slow and have to turn sharply to get into a spot. I asked the shop to listen for it and after much experimenting they heard it. They believe it is one of my ball joints. The owner said probably the stablilizing arm I believe. He said it is a common repair. He told me to wait till it happens more frequently so they can determine for sure what it is. He said it could be a number of things including the rubber wedge mentioned in an earlier post.
I never had this noise until the bearing replacement and the mechanic is a good freind of mine. The owner says it isn't uncommon that after a car is put on a lift to be worked on that a unknown problem becomes more noticeable because of the way the springs and wheels and such hang. Simply, some things move a bit and I believe this guy.