Rumbling/Grinding on rear wheels?

[WM-S12]

recently i've started hearing this awful like grinding and rumbling coming from the left rear wheel. It sometimes comes and goes but it tends to pop up specially when I hit a bump, however the moment I use the brakes it stops the noise I've driven home using the heel toe method just to stop it irritating me.

It then doesn't even happen some days. I'm slightly curious as I got new discs on the back a few months back, alloys which are open to the elements and well I was out on some loose gravel mucking around. Any chance a stone or something has lodged up behind the back of the disc brake. It has the metal shield at the back so that's one guess as to what it is.

Im pulling my hair out, it makes a noise then it doesn't, when it makes a noise I apply the brakes and it stops happening. Diff....brakes. I've ruled out suspension. Not so easy to diagnose a problem using sound on a forum

frustrating