They are digital servo's so the servo is constantly looking for center, so it is common for them to be making noise very often. If you lift the front tires off the ground does the servo go quite at all. If not there might be a problem. Take the servo out of the car and see if it is still constantly making the buzzing noise. If it still doesn't go quite there is a couple things you can do:

1st - Phone Hitec and tell them the servo never stops buzzing even when it's out of the vehicle with nothing attached. They might offer a exchange for you.

2nd - Run the thing anyways till the servo goes and then get Hitec to replace it.

Hitec's warranty is one of the best in the hobby, and a lot of the time you don't even need a receipt. Just send the servo with a letter saying what is wrong and you return address. Of course if you have the receipt send it with it as well. But every person I know has got the servo replaced or fixed even if they didn't have the receipt of purchase.