This squeaking seems a common problem. Mine does it. The fault in my opinion is the non rubber grease lubrication of the anti roll bar at the rear, especially the drivers side where the bush is over the exhaust. I do not think the bushes are lubricated on assembly and they "dry out". Do you find that the squeak gets less or even goes away in wet weather? Mine does.
I did report it under warranty and they said they had lubricated the bushes, but when I inspected the work they had just sprayed the area with lubricant. It did the trick, but was only a temporary measure as the squeak returned. I have resorted to spraying motorcycle chain lubricant on them myself, which works for a time.
The problem is the exhaust and fuel tank will have to be removed to properly put rubber lubricant in the bush which is over the exhaust. A BIG job for a little, but annoying, problem......Should be done on initial build!