What do people use to stop the top harness mounting buckles rattling ? Its very noisy when theres no one on the passenger seat. I should image theres just some sort of plastic or rubber washer that ptakes up the space and prevents metal on metal contact.
Ive been searching on Ebay under combinations of; seat belt mounting washer, harness mount anti rattle shim etc but cant find anything that seems appropriate.

Typically they have a thin special spring washer (wavey sort of thing) that fits between the harness end and the spacer bush. You should be able to knick some off seat belt mountings at the scrap yard.

You can either use wave washers or rubber O rings.

STAINLESS STEEL M12 12mm WAVE WAVEY SPRING WASHERS PACK OF 10 | eBay

METRIC O RINGS BLACK NITRILE RUBBER 12MM ID X 2.5MM THICK QTY 100 | eBay

Many thanks, ordered.