Bang, Bang?

[droptop]

I finally got my spider on the road on Tuesday evening and found an occasional sound from the back, rear right, I think. there is a couple of bangs, not backfires, like if I ran over some debris on the road and it struck the bottom of the car. This is followed by a sound like I'm dragging a piece of metal along the road. Now, all this happens in a couple of seconds and I have found nothing on the road, so I haven't lost anything, and anyway it has happened three times in total. It appears random as the circumstances and locations were different each time. I know it's not the exhaust and looking under the car reveals nothing.
No amount of bouncing or shaking can replicate it when the car is stationary.
I am guessing suspension cos there really isn't anything else back there.
As I write this, I just wonder if a bad engine mount would be causing excessive movement of the exhaust system and causing the transverse box to hit something? I am not aware of an engine mount problem, but something is happening.
This has occured when I am (A) on my own, (B) with a front passenger and (C) with three passengers (the two in the back had to be dragged out as cramps and pins and needles had set in and I didn't want locking them in the workshop over night)
All thoughts and suggestions welcome as I have a 150 mile round trip to my first show tomorrow and I am determined to go.

[mark]

Hi, Have you checked the rear disc back plates ? If they have corroded and come away a little, that could certainly give the metalic scraping sound as it rubs against the disc. Good Luck. Mark.

[lanciamad]

Spring condition?, top mount ok?, piston rod nut tight?, is the spring seated correctly on its platforms?, are the rear transverse arms straight? If all thats correct then chances are its not the suspension.

[droptop]

After the road trip, I discovered that the front engine mount had failed and as I needed one in a hurry, I have made one by shaping and welding a plate to the lower part of the two steel side plates of the original mount about 25mm up from the bottom and attaching a round generic mount of 60mm dia and 50 mm height which has given me the height of the original mount when it was together.
Now, i haven't driven the car on the road yet, but having moved it around the workshop and yard, the gearchange has improved and there is a hell of a lot less vibration, so some obvious benefits are immediately apparent.
My only concern is that it will be too stiff in relation to the other mounts and overwork them to compensate.
Anyway, a good drive is in order tomorrow and we shall see.
I feel the mistery sounds may have been caused by this situation and it can be very hard to determine the actual location of the source of knocking and rattling in a moving car.
i once almost scrapped a Mitsubishi as we were convinced it was going to fall apart in the front due to banging and knocking on acceleration and braking. It eventually turned out to be a 32mm 1/2" drive socket under the carpet under the front passenger seat!
Anyway, I'll soon know   

[droptop]

Update on original post.
Just looking over some of my old posts and reviewing the story so far. I never updated this one.
I think it is important that we inform others of solutions to problems we encountered as this shared knowledge, whilst sometimes of little interest unless a similar problem is encountered, will serve to make life easier for all of us trying to keep this marque on the roads and also I feel it's important to acknoledge the decency of others to respond to requests for help and the time taken by them.
Anyway, the engine mount is in its' 2nd. year and as it turned out, was not the answer.
Metallic bangs and clonks can travel and often be misleading, and this was no exception.
I was blaming everything including the exhaust, but eventually found the cause a couple of months ago.
It turns out that the rear axle has two arms running parallel to the main beam and one of those had been used to jack up the rear of the car at some point, deforming it and causing it to come into contact with the main beam as the right hand wheel went up or down.
The resultant sound caused me to visualise a seized shock or broken mount as the car wouldn't compress or rebound to anything like the same degree as the other side.
Long story cut short, I prised it back roughly into shape with a flat cold chisel and was amazed how pliable and flimsy the metal actually is in these components.
My intention is to remove a good pair of them over the winter and fabricate new ones from reasonably thick-walled tubing as I know the bent one has to be affecting the toe angle of my rear wheel and I get a great degree of satisfaction out of making my own components, especially if they work.
As I have said in previous posts, the car would still be in the garage if it wasn't for all of you who have provided me with answers and parts, so thanks again!