The inner layer of the side wall cracking and letting air psi in between the layers.
This usually happens from hitting a curb or hole or something like that. Thou, there are usually other signs of hitting something. Usually there would be some kind of marks on the rim, the other side wall and sometimes on the tread it self.
They (customer) came in complaining about a vibration but would not admit to hitting anything. There was a mark on the inside edge of the rim so I know they hit something.
I have another job coming up....an IS250 that someone stole the wheels off of and left it sitting on the ground. The owner had a tow truck bring it in and let me tell ya, that tow truck effed up the under side of that car. I will get some pic when I start the job, tomorrow or Monday.
Just taking care of the "safety" items, his parts bill came to just over 4k and about 13 hours of labor.
Come to find out the IS owner has no insurance or license so all out of pocket $$$.