with metal gears (Markits or whoever) if the worm and gear are perfectly alligned at 90 degrees then gears just have a slight whirr, but if not then theres a sort of gnarly whirring sound - if you've heard it then you recognise it. With some they can be quiet as anything with just chassis but add white metal body or a load then can become gnarly whirring - plus wheels slow considerably. Also often slower and noisier running one way compared to other. Have checked alignment - by eye, have checked theres the easily the rollup paper space between gear and worm, have checked gear box sides are parallel and 90 degrees to motor. Is there anything else that could be done. Someone did mention giving a run with toothpaste to grind sharp edges, but would that fix this ?
Cheers, Simon