Strange noise after diengaging handbrake

The handbrake on my disco is slipping but (which may be related) I've just noticed that when I've yanked the brake on to stop the disco sliding backwards, then release it and set off forwards, there is a chug chug kinda sound. The sounds doesn't seem to increase in speed as I go faster but if I blip the brake pedal it stops and all is good again.
Any ideas?

Disco is going in for new handbrake shoes, reversing light switch and cambelt (and aux belts) tomorrow so it may go away but I'm intrigued as to what this could be.