Not to be an alarmist, but when you're under there take a look at the frame too. Metallic creaking noises can also be made by frame damage (cracks, rust, etc..). Not as likely as the other stuff though, probably more like worn out bushings. Just because they're only 6 years old doesn't mean they aren't bad, factory stuff lasts a lot longer than aftermarket rubber parts from overseas, and if the car has been sitting the rubber may have rotted. Also, it may happen just as much at higher speeds, you just might not be able to hear it. Classic cars like ours tend to get noisier as they go faster (wind, engine rpm, other rattles) and can mask the unusual sound you're looking for. Happened to me more than once!