Q: Knocking sound under car

Hi there. Either the tires are out of balance and causing the front end to shake and make the steering parts move around or there is a loose steering or suspension part that is moving around as the vehicle is going over bumps on the roadway. Also, the air dam under the vehicle could be loose causing it to make the noises that are heard at highway speeds. Look for any loose items that could make a sound when there is excessive air passing around them. If you need further assistance with your vehicle shaking and making a noise, then seek out a professional, such as one from Your Mechanic, to help you.