Astra Rear Brake Drum Problem

Hi, I have a 2003 Model Astra SXI (3 Dr)

I bought it in march from a VX local dealer, covered with Network Q etc

At around August time the rear drums started to make like a liquid/rumbling
sound everytime I applied my brakes from a cold start when I used them to
brake at a few junctions, after about 5 mins from starting the noise
disappeared.

It kept doing it and started to get worse, it didn't affect the braking but
caused concern, after all they are brakes and they shouldn't make noises.
I booked it in at the dealer about a month ago where I purchased it from and
they checked it out, confirming that the rear drums needed cleaning as dust
and dirt had got in their.

They charged me £22 for doing this as this wasn't covered under the Network
Q policy.

It's been fine for the past month but today it started to do the same again,
the same as before but more quieter this time, but I know it will only get
worse.

I was told that my brakes where fine when they inspected it, so I don't know
if it's the same as last time.

I don't know if they have to strip the drums down to clean them or do they
just spray some kind of cleaner into the drums? is this a common problem on
non ABS Astra's?

Can anyone shed some advice?

Thanks

Pete