Could you provide some more information such as:
- Does it knock at regular intervals or irregular?
- Does it change with RPM or engine load?
- Can you pinpoint if the noise is from the front or rear of the vehicle or which side it's coming from?
- When driving above 45 mph, have you driven near a cement barrier or curb to make sure the noise does stop? Sometimes you may not be able to hear it in the cabin but it's still happening.
- How long has this been happening?
- Did anything change with the vehicle before this started happening?
- Is this a 2 or 4 wheel drive model?
Any information on what you have done to try and pinpoint or stop it would help. There are a lot of things that can knock underneath the vehicle, especially on an older one.
From the info you have given, I might say it could be a wheel bearing....I've seen having different types of tires or tire brands between one side and another cause a knocking like noise before too. Really hard to say but if it gets worse when you turn left but not right then I would look somewhere around the wheels.
