It's probably a wheel out of balance, or some other front end issue. I would guess that a previous renter hit a pot hole or a curb and threw something out or put a bubble in the tire, etc.
I would also guess (and it's just a guess without knowing the cause) that it's probably not covered under the car warranty if they consider it to be abuse, and because it's a Mercedes, repairing it probably won't be cheap.
You probably can't just show up, you would probably have to make an appointment for service, and you could ask if they would deal with you or only Hertz.
I would not consider the car safe to drive unless the cause is identified and they tell you not to worry about it, you could have something ready to pop or break off at any time.