My car is grinding and shaking at around 42 MPH. It stops when I put it into 3rd gear.
If your car has a manual transmission, the some of the hard parts inside the transmission may be damaged. A manual transmission has gears, bearings, brass synchronizers, etc. Some of these are used together in certain gears and not in others. A group of gears that are damaged might be heard in lets say 1st & Reverse, but not 3rd & 4th. Some transmissions have an access port for internal inspection and some do now. I suggest seeking the help of a mechanic to help properly diagnose this issue.
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