[Lynn Eades]

I;d say the bolts in the tube under the engine are loose. There are two up front, near the division plate, and two at the rear in front of the bearing.
Movement is happening there at one end or both, and your gear lever gate is stopping you travelling side ways enough.
If it has been loose for a while, wear will occur as David has said. Select first or second and tighten the bolts.
There is a reason why the Australians did away with the gate..............