Just thought I'd add the problem is usually the reverse idler gear shims in these gearboxes, as was touched on at the beginning of this thread.
A total stripdown is the official cure - forget it for a rattle at idle!
See the last post in this tread:
http://www.peachparts.com/shopforum/diesel-discussion/360329-getrag-5-speed-85-w123-300dt-2.html
Mine is now silky smooth and silent at all speeds and I'll be leaving it just as it is! Thanks again Paul.