After the car has run for 5 or so miles (not from startup) I get a ticking noise from the rear of the car - it sounds like one of the tyres has a stone stuck in it - but they haven't. The noise starts at about 35mph and is audible until the wind noise cuts it out at about 50mph, so I don't really know if it disappears.

Today I noticed the noise has delveloped a little: now when I speed up to 40 or so and then come off the throttle and let the car coast it makes a rattly noise like I have a bit of wire wrapped round the driveshaft and its thrashing at the underneath of the car. :o( It all sounds quite nasty and I am worried about driving about with such grim noises coming from my baby.

I have looked under the car for any stray bits and bobs, but its all normal looking. Should I be checking for any lubricant levels? Theres no sign of leakage from anywhere near the rear wheels. I wondered if there's something up with the differential...? The car is an automatic (6/00)
It's the word "auto" in your signoff that could give a clue here - I used to have a Cavalier automatic that made some odd clicking noises. Got rid of it before anything became serious though. Not much you can do with a modern slushbox other than change the fluid, and filter if accessible. (6/00)

Have someone step on the go-pedal while you check under the car for leaks around the exhaust. Make sure the car is safe and isn't going to roll over you while do this, of course !!! My guess is that you've either got a rattling heatshield (built onto and above the OEM exhaust), or that one of the welds on your exhaust is leaking (I had the same tinkling effect when the weld on my Thermal R&D exhaust stated to leak). Your coasting decription reminds me of this problem. I had the weld re-welded but it leaked again two months later. I've just had it welded again, and had a little bracket welded between the silencer and the pipe entering it, to help stabilise things. You'll quickly see/hear a leak in the exhaust.

If it aint that, then you should get your diff checked out - I know nothing about those.

Are you sure it isn't from the engine compartment ? Ticking can be a sign of bad ignition leads. (6/00)
Can you chock the wheels, jack up the back and try turning a wheel by hand? This might help track it down. It probably worth pulling the rear wheels off and having a look around. You could also trying changing your diff oil, but if the diff. is making a noise like this there's more wrong with it than new oil will fix. :-( (6/00)
....yes, could be a broken wheel bearing cage. (6/00)
Anyone have hints on how to check the wheel bearing cage? If I know what I am looking for I am pretty competent, I just have no idea what to look for yet. *sheepish grin* (6/00)

Could be exhaust as said .. these things sound alarming sometimes...

I guess we are talking 1.6 car here .. with known diff weakness .. so could be diff/prop/driveshaft.. you need to go shove all the shafts around as hard as you can by hand and feel for any odd effects.. the ultimate would have to be a highly dubious practice that I cannot recommend ; put car on axle stands and run it up to 'noisy speed .. and listen .. you might have to load the car by lightly pressing the brake pedal with your hand to get a repeatable noise.. as I say this is a dangerous/dubious thing to do. If you changed the diff oil and it was on the way out .. you would tend to get more metallic bits in the oil .. but you always get some anyway so this is highly subjective .. oh well .. changing to clean hi-performance oil would do no harm and may quieten it for a while... (6/00)