Toyota Corolla 2002 Problems

NO IT won't cause wear on the gearbox, but it's not a good idea on safety grounds. He's got no chance of getting out of the way should something happen that puts him in danger while he's sitting there in neutral. It's much better to be in drive with an escape plan just in case.

You can fit LPG to your Corolla, but it is recommended that you use an injection system because of the risk of backfiring with the older system and the damage that could cause to the plastic intake manifold.

The person repairing your cylinder head should have used LPG -  compatible valves and valve seats; had they done so you would not have any further trouble. We have been asked about the upper cylinder lubricants you refer to many times, but we have never been able to establish their worth beyond the claims made in the brochures etc. Running on petrol will only be effective on valve train degradation whilst running on petrol; periodic petrol operation is to recommended to protect the petrol system components, keep the fuel volatile, etc . Keeping the LPG system in tune should be in line with the manufacturer's recommendation for periodic scheduled maintenance and should be provided in the operators manual for the gas system. The petrol system should also be serviced in accordance with the manufacturer's recommendations. As a precaution, have the valve clearances checked every 10,000 km.

The Dunlops are regarded as relatively quiet tyres and are unlikely to be the cause of your complaint unless there is a misalignment problem with the rear suspension. If there is, the tyres could be wearing unevenly which could cause a noise problem. Other than that, the most likely cause is the body structure of the Corolla and its lack of insulation which allows the noise to be transmitted through to the interior. In that case, changing tyres is unlikely to achieve the sort of improvement that would satisfy you. Check the rear wheel alignment and adjust if needed. Other than that, I reckon you're stuck with it.