Its an old thread, but feel compelled to share the result of my rattle quest!
I had this really annoying rattle at low speeds mainly, it sounded like muffled metal on metal knock, and was definitely coming from the rear, both left and right.
The car is an E86 3.0Si late 2007 with 140K on the clock.. But mint in every respect other than this infuriating knock!
On the quest I replaced:
1. Rear top mounts for Meyle HD ones (more on that later)- Knock was still there :thumbsdown:
2. RTABS - Knock was still there - On the plus side the car tramlines less
3. Put on Bilstein B4s - Still there

4. Front control arm bushes (More an excused and on a whim) - F'n rattle was still there
5. Sway bar bushes - Still there
6. Sway bar drop links - Still there
I then went back to the start and did some research on the Meyle HD top mounts, and found there are a terrible design which keeps the lower washer from sitting on the rubber bush... So this causes a knock. See the photos attached
8. I bought some lemforder top mounts and the rattle gone!
Until amount 100miles later when it returned! I then inspected the top mounts again and found that because I tried tightening the centre nut as best I could (Tight as ...) this had pulled the shaft up through the washer, which meant between the washer and the centre piece of the top mount there was a small gap, probably 1 mm which even when fully tight, it was still causing a knock!
I then thought I'd get some awesome power flex bushes... This would surly solve the problem! - Wrong!!! This was only slightly less bad than the Meyle HD top mounts.. :thumbsdown:
I then used the Dremel and made sure the lower centre part of the Lemforder top mount was wider than the shaft edge to ensure the rubber mount sat on the lower washer.
I gave this three ugga-duggas with the rattle gun, while gripping the shaft with mole-grips, and put everything back together and voila!
I only dread to think the cost of all of these changes if done by an indie.
Anyway, I hope this helps, and give a thumbs up as this is my first post ever. Im so chuffed I even made a short video.
[youtube]https://youtu.be/uCG-0jnJFSI[/youtube]