Ive done a good old search but am still perplexed as the exact problem never gets an answer.
In the last month or two this noise coming from the rear has done my head in.
It only happens on some surfaces, always at low speeds, say, coming to a stop or inching in traffic queuing. I am getting the usual WRX bug suspension clunking but this is different.
I have replaced the rear diff fluid twice in a year (I changed shortly after buying then again last month to check for filings etc) and the rear pads are grand. Discs are to be replaced as they look bad but arnt overly worn.

My handbrake is quite loose so am going out to the car now to tighten it and to jack the rear, free rotate each wheel and see if I can detect where it is coming from.
When moving slowly in traffic it makes the noise from the rear and gives the sensation of driving over fresh snow as you can feel the noise as well as hear it if that makes sense.

Help!

 

Sounds a bit like my problem although mine is from the front. My s**TTY mechanic knocked my abs sensors when changing my discs and pads now every time I stop even at slow speed the pedal shakes and get a weird grinding noise. V irritating. This probably isn't what yours is but worth a check?

 

the noise you are hearing is normal i have had it on all my blob and hawkeyes it does this untill the rear brake pads get heat in them.(then it should go away)

 

When i had grinding coming from the rear of mine it was the metal wear indicators on the brake pads.

 

the metal wear indicator does cause grinding but this is really loud??

 

Brake pads? Its more of a lower grinding though, hmm, I replaced them less than a year ago (less than 4k) but one may have broken up... will have a looksie cheers.

 

Sticking piston or slides?

 

Prob not that then,perhaps a spring or something has come adrift on the shoes.

 

I have traced the problem to the rear driver side shock, strange one as it isnt the usual clunking. cheers for the comments.