It's good to know she doesn't pull towards cars. Some dogs have an intense desire to chase things, including cars. It sounds like she isn't combining her fear of cars with a desire to chase and/or bite them. That's good.
This is going to be a long post. It won't, yet, give you a solution. It is just to help you prepare a foundation for your solution.
I already suggested, as a first step, to avoid putting her in situations where she rehearses her fearful reactions. Of course, avoiding noisy vehicles forever isn't realistic. Think of this first stage as a mental holiday for her. It won't last forever, but it will help her regain her equilibrium. While she is having her holiday, work on playing some training games with her. I would also reinforce playing with a tug toy, if that's something she enjoys. For training games I would suggest Look At Me, Touch My Hand, Face Me, Move to My Left Side, Move to My Right Side. Teach her each of these commands, and reward her with a treat and happy praise each time she complies. Then increase the challenge for her: practice walking while she follows along looking at you, while following your hand, while moving between your left side and your right side. Switch between tasks. Ask her to look at your face. Reward her, and ask her to move to your side. Reward her again, and ask her to face you. Make sure these games are fun for her. They should be a way for her to feel like she's the center of your attention, and that she's the best girl ever.
Tangentially, here are some important training concepts to think about.
One is called
trigger stacking. What this means is that the pressure from a series of minor stresses builds up, like shaking a bottle of fizzy water. She may not react much to the first noisy car, but it has upset her slightly. Each new car upsets her more. Finally she has a meltdown when she sees yet another car. This new car isn't doing anything different from all the previous cars. If this was the only car she had seen that day, she would be fine. It's just that seeing multiple cars has pushed her over threshold, and she's too stressed to function properly anymore. Once she hits her limit, it can take hours or even days for the stress hormones to leave her bloodstream. The best thing to do is to let her rest quietly at home, perhaps with a few quiet activities like chewing on a bully stick, until she recovers.
Your task, regarding trigger stacking, is to observe your girl. Learn what your her limit actually is. Returning to the metaphor of the fizzy water bottle, how much shaking will make the lid pop off the bottle? One car? Three cars? One car and one noisy truck? Then, start counting the number of triggers you encounter on your walk. When you start approaching her limit, turn around and go home. For example, one of my dogs is reactive towards other dogs he sees on our walks. He is fine for the first two dogs. Then he starts whining. After five dogs he is likely to have a full blown screaming meltdown. Therefore, I will turn around and go home after we see three dogs. He's fine once he has a nap; we can go out again later that day. If I ignore his needs, allowing him to get into a full blown screaming meltdown, I need to keep him home for 1-3 days while he mentally recovers. Your girl has a similar limit. You need to observe her and learn how to stop before she's overcooked.
The other important concept is the
Three D's. These are
Distance, Duration, Distraction.
Distance. Perhaps your girl is fine if she sees a car moving at the far edge of the field, but she can't handle being on the edge of the road when one goes past.
Duration. Perhaps she is fine if you take ten seconds to walk along a three meter strip of road, she can't handle walking for five minutes along the edge.
Distraction. Perhaps she can handle a relatively quiet Toyota Corolla passing by. A van towing a trailer full of rattling landscaping equipment is too much.
As you increase the difficulty of one D - decreasing the distance, increasing the duration, increasing the distraction - you will probably need to decrease the difficulty of the other two D's in order to keep her under threshold.