I have a 2010 NC 3.5, and I bought it from the local Mazda main dealer about 3 years ago. Since then I have noticed that when driving over even a small bump, I can hear a knocking noise coming from both back wheels (I think!). It is the same sound hitting two hammers together. A very metallic click. (before you ask, yes, I have removed the jack from the boot!) I took it back to the Mazda dealer under warranty and they said that it was a common fault and they needed to replace what they called the rear suspension caps. That didn’t solve the fault. Now not under warranty, I took it to our local repair garage who have a very good reputation, and they couldn’t find the fault, they checked everything and said it was all looking as it should, they couldn’t even hear it when they test drove it but said that the exhaust heat shield was loose and that was making the noise (probably). They remove the heat shield and test drove it with me in the passenger seat and looked for small bumps, drain covers to drive over and it seemed to be fixed. Only it isn’t! I still hear the same noise when driving down my road which is quite bumpy.
I am puling my hair out now, does anyone have a similar experience or know what the problem may be?
Drop links maybe, this link may help
Best guess and diagnosed on my mates NC was the rear drop links.
Easy and cheap repair. 
(Maybe not that of course).
Cheap enough to change rear drop links, getting them off easily is sometimes a challenge. You’ll probably end up cutting some of the bolts as the rust up, easy enough though.
Moving on if that doesn’t fix it, it didn’t on my last NC Sport with Bilstein suspension, I changed out the whole suspension, rattling cured. Probably not ot what you intend doing but I had planned a suspension change and lowered NC anyway.
Could be brake pad rattle.
I’d normally do droplinks first as well as they’re the cheapest. Also check the rear box for the exhaust isn’t moving about too much on the mounts.
Thanks guys. I have had a look under th erear of my car and it appears to have already had two brand new drop links already installed. see pics. So its not that.
Pad rattle. To confirm it, find a rough bit of road and brake gently as it’s rattling. The rattle will stop, just like that. Mine has done it almost since new!
I’m sure that it’s just the camera angle but that left rear spring doesn’t look too clever. It’s not got a broken coil at the top has it? It looks a bit banana shaped.
You’re right. Top doesn’t even look seated and shock doesn’t look centrally sat in the spring.
Another vote for getting that rear left spring looked into.
Also, the ARB bushes look quite tired, they are a cheap and easy replacement so may be worth while to change them.
Had a noise like this on my NC, mainly heard when roof down. Think it could be knackered shocks as the noise has now stopped and the shocks leak!
Thanks guys. All points taken. I have it booked in to get it all looked at now.
I had similar problem. Was the rear shock absorbers, which were replaced and hey presto, noise went away!
This is exactly the problem.
The excellent photo is clear.
I’ve zoomed into it to show the detail of the broken-off end still secure on the seat, but with the spring now displaced and resting to one side of the broken-off end.
Oh my god! You are spot on. I have just gone under the car again with my camera and got a pic from a better angle. Thank you so much for your help. See photo…
Great spot by those above.
Shame the garage/s you have been using couldn’t spot that well before now.
As the last 3 years…. Hmmm.
My recently acquired ‘06/47k had two broken springs at its last MOT. The previous owner hadn’t noticed anything amiss and he was a Diesel Fitter by trade so you would expect to be mechanically attuned.
Are broken springs common on cars? Seems odd to me as they are designed to absorb shock and I’ve never had one in fifty years of bikes and cars
The more potholes in an area, usually the more broken springs.
That said I believe constant compression such as being parked up a kerb (for a long time) could lead to one developing through lack of use.
I think some drivers just don’t pay enough attention. I had a workmate turn up one morning and asked me to have a look as his car ‘felt funny’. One rear spring had broken clean in half and he had no suspension at all on one side as a result. I managed to pop home and get my stuff to change the pair in the car park at work but if we couldn’t he was quite willing to do the 45m drive home with it broken 
.