light thunk coming from rear

[JT///M3]

this sound only happens when the car rolls backwards (in reverse or neutral, like when backing down my driveway) but doesn't always happen. Its very light and can be barely audible from the inside of the car. what could it be?

TIA

[s_ribs]

Must be the age of these cars. Check the rear shock mounts. You'll have to pull the carpeting up in your trunk to check. I would go straight for the Ground Control RSM, E46/Z3 will wear out GC won't. Real easy straight-forward repair. Should only take about an hour to do. You may not hear it at speed because of other noises.
Seth

[themadhatter]

120k+ on my buddies '95 M3 and no odd noises.

there must be something loose or worn in your car.

common wear and tear problem areas on the E36 are:

rear toe bushings
rear shock tower mounts
differential bolts (I should know about this one :()
torn rear subframe mounts - usually caused by not keeping an on your bushings when they get older
guibo - this is your rubber flexdisk located on your driveshaft

all of these items are DIY if you know how to turn a wrench. :)

I'm sure there are other issues as well but these are the most common for the E36 chassis regardless of model.

-Ron