Android OS + others data usage

Hi,
I hope someone has found a solution to this:
device Samsung Galaxy s duos GT-S7562 Android 4.0.4. Android os + others is using excess data through 3g connection.
'Back up data to Google servers' is off, sync is off but it still uses 1mb in 30 minutes when the phone is idle with active data connection.
The phone is rooted and I've tried blocking all the apps using avast firewall and droidwall but the Android os + others is still using mobile data. This is frustrating as I am on a limited data plan.
All the data tranfered is background data and there's no option of turning off background data on Android os unlike the other apps.
Turning off background data for all the apps is not a solution since some apps like whatsapp need background data.
Has anyone found a solution yet?