Good idea about tapping around with rubber mallet. I will try this tonight. 

Yes. The springs are stock. 

I had a suspension rattle in my '10 Clubman, before I traded it in ...

I found it by tapping around on the suspension parts with a rubber mallet until I produced a noise like the rattle. I don't remember what was rattling, but I found it and tightened a bolt/nut to eliminate it.

The only thing I can suggest is to get some silicone lubricant spray and selectively spray a specific area of the rear suspension, then take a drive to see if the noise changes.  If no change, spray a different area and repeat. This should help isolate the problem area.

If the sway bar is rubbing, there should be some signs of contact on the sway bar or brackets.

Are you using stock springs?

ken

Driving me nuts! PLEEEASe HELP..... 

Just put new Koni FSD struts and WMW 25.5MM rear anti-way bar on my 2003 R53. I am getting a rattling coming from what sounds like the passenger rear area when on a rough or bumpy road. and the squeak sounds like a baby bird chirping... Been a week and so far I have....

Tightened the: 
-sway bar links at the top.
-Strut mounts top.

Shifted the passenger sway bar mount forward as far as it can go and tightened bracket... and rolled the body seam next to the bar/mount under to create more space (with a air chisel) ... this helped a little with the rattle.. 

I'm stumped at this point... 

The rattle/squeak have the cadence of a spring like rattling... if that makes sense. It doesn't just rattle with the bump.. it Has a little bit of a reverberation afterwords. Seems like it is the swaybar vibrating and hitting something... or rear links loose... But I'm not sure what it could be at this point. The spacing is very tight with this bar. It is possible I will have to go with a smaller bar... suggestions welcome.

It could also be that the strut was not properly seated with the spring??? mot sure at this point....

This car is a daily driver.. But I do visit NC mountains annually and push it to its max.. and I would like to do a track day for fun once a year or so... so I don't need race performance but want to eliminate/reduce under steer..