if there were any sort of award for complete lack of information i think you would have just taken it.
from your initial post i was able to glean that there was a sound coming from the left rear wheel, with a little research based on the info provided i discovered
Sound: Vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being detected by human organs of hearing.
so this will be whats going on somewhere around that back wheel ?
fortunately you've added " it happens when slowing down from faster speeds" which sadly gives us no further information because, ultimately thats what brakes do.
please come back with something useful the model and age of car would help to begin with, also a description of the sound other than 'Strange' is it grinding, squealing, banging etc
seriously??? "I have a sound what is it?" thats all you've basically asked