I've had a knocking noise coming from the rear of my Signum for two months and I just can't find the cause. It sounds very similar to the noise of the exhaust hitting something, or perhaps the 5"x3" stainless tips hitting the cutouts in the under-bumper skirt, but read on and you'll see it's not that. Itseems to come more from the rear N/S than the O/S. This is what I've tried so far to solve it:
1) Removed the bolt-on exhaust tips to see if they were hitting the bumper cutouts. Noise still there.
2) Got under the car, found middle section of the heat shield hanging down where it had rusted away around the bolt, bolted back with an oversized washer. Noise still there.
3) Checked all the rubber mounts for the twin back boxes and tied the larger backbox to a bracket on one side to stop it swinging as much (even though it wasn't hitting anything). Noise still there.
4) Found a bit of the heat shield that was touching the back edge of the wheel arch liner and bent it out of the way. Noise still there.
5) Further up the exhaust, found the bracket that holds the flexi section to the rubber mounts had rusted off, still connected to the rubbers though, so it could have been hitting the exhaust. Removed it. Noise still there.
6) Jubilee clip that I'd put around the flexi 4 years ago to stop a rattling noise at low to mid RPM had rusted and was sitting loose on the flexi. Took it off. Noise still there.
7) Flexi looking a little tired, but not too much, but had a new stainless flexi from MIJ welded in place. Noise still there.
8) Had a garage look over the entire exhaust, for signs of damage or knocking, they couldn't find anything. Noise still there.
9) Found that the rear ARB droplinks were looking cracked so replaced them (and wrote a How2). Noise still there.
10) Outer bushes on the upper control arms were too soft, so replaced them (and checked the inner bushes - they were like new). Noise still there.
11) Compressed rear N/S spring using Laser compressors and undid outer bolt on lower rear control arm to be able to see the upper and lower rubber spring seats for signs of perishing. All fine. Noise still there.
12) Replaced the ARB bushes. Noise still there.
13) Had a garage look over the rear suspension and springs, all intact, wheel movement pronounced as "solid as a rock". Noise still there.
14) Tightened the tailgate latch in case the boot lid was rattling. Noise still there.
15) Took out everything under the boot floor. Noise still there.
16) Rear wiper spindle moving just a little when wobbled by hand, so took off the tailgate interior trim hoping to find the wiper motor wobbling around, but it was held very firmly in place. Noise still there.
17) Rear number plate bouncng up and down on its screws when moved by hand, and screws just spinning when I tried to tighten them, removed number plate and took car for a test drive. Noise still there.
18) Took out rear brake pads on N/S hoping to find the anti-rattle clip broken or even the pad snapped in two. All fine. Noise still there.
19) Disk shield wedged against the bottom edge of the rear caliper. Bent it back using pliers. Noise still there.
I can't think of much else to try, and the noise is getting worse and is almost constant now. The only two things I can think of to try are to check the O/S rear caliper like I did to the N/S (but the sound comes more from the N/S) and try replacing bush after bush in the rear suspension - the ones that hardly anyone ever bothers to replace because they usually don't fail - could be expensive and certainly very time consuming, and probably won't solve the problem.