80 series chatter

just wondering if i can get some thoughts as to the origin of this chatter:

i have an 80 series GXL 1HZ with the part-time conversion fitted (ie: centre diff lock becomes 4wd button) HF2A transfer case

when in 1st, low revs, and foot off the gas (idling revs) it makes an obvious chatter noise. as soon as i speed up a bit it disappears completely, but take your foot off the gas (ONLY at idling speed) and it returns. revs over idling and anything up to highway speed it is perfect. no difference between 4wd or part time. No difference between high range or low range.

I am aware that (most) other older 80 series landcruisers have the same noise, and that it is sopposedly not a problem but i'd like to sort it out none the less. I figure its a common issue as I hear it in other vehicles at the servo for example.

when walking over rocks in 4wd (foot off the gas), the noise is loud and comes and goes as the wheels walk over the rocks.

front and rear diffs are perfect. no freeplay in shafts. no problems at all changing gears.

I suspect it is something upstream in the transfer case???

Any comments from others who have had the same noise????

Thanks in advance chaps.

husky