G'day,

 I posted this in the 'touring' section because I have a tourer - but on second thought, this post more logically belongs in this forum.

My bike has recently developed a noise in the front brake. It's a low piched squealing type noise (almost like a 'moaning' sound) that only happens when I am braking really lightly - like when I am coming to a slow stop.

Checked the pads and they look like they have plenty of 'meat' left on them.

Any ideas as to what causes such noise and how to stop it?

 Probably something as simple as a build up of brake dust. 

Usually caused by the brake pads chattering on the discs.
Probably worth doing a bit of maintenance.
Pull your brake pads and deglaze them, and give your discs a good clean.
Also give the pistons in the calipers a blast with brake cleaner.

can all so be caused which is the most common problem ,which is the contact between the caliper piston and disc pad as you brake the movement between them both
you can buy this stuff from repco/coventrys called brake squeak its a bit like salistic in a orange bottle ,but I would also do some brake cleaning while your there as
speedzter said just watch any paint work with the brake cleaner depending on brand it can take paint off or dull it

This may help you.  See item 5.    If you have'nt change your pads before , I'd skip item 4. Just give the caliper a good clean.