Weird AUII XR8 rattle

[undertow]

Had this noise since I bought the car a few months ago. It hasn't gotten any worse or caused any problems, but it's irritating and I figure someone else might have had this before.

My AUII XR8 (auto, 180,000 kms) has a weird light metallic rattling sound while driving that seems to come from the gearbox area or rear of the car. The best way I can think of to describe it is like hundreds of crickets chirping together! It is not loud but noticable with the windows down or up on a smooth road with the stereo off. Does not rattle when the car is stopped, and starts to be noticable at about 30km/h, then stays about the same volume from 40km/h through to 100 (at which point the wind noise starts to drown it out). Does not vary with engine rpm/gear changes, but usually gets a bit louder when accelerating gently and quieter on overun. Can still be heard with the tranny in neutral. The pitch/tone doesn't vary with speed either, it stays about the same.

Sounds a bit like a vibrating heat shield or cat, but it doesn't rattle when I rev the engine in neutral. Does not vary in pitch with speed, which suggests not diff bearings. Does not vary with engine rpm, so it's not torque converter bolts etc.

My thoughts are maybe CV joint, wheel bearing or uni joint, but I would expect these to vary in pitch/tone with speed. Anyone come across something like this before?

Thanks for any help.