Hi Keith, I found the same with one of mine on the E30 last year.
Turned out to be a partially seized slider due to the rubber boot
having split and let water and dirt in. I dismantled and cleaned it up,
and rebuilt with lithium based grease and a new boot. Did the other
side too for peace of mind. It's been fine since. If you take the pads
out you'll be able to feel by hand where it is seized. Get some
threadlock in before you tackle the job, you need it for the slider
bolts.
--
Mark