I know you'll have checked, but does the gearbox have enough oil in it ??
I'm thinking that if it happens more when slowing and when cornering, a low level might increase the chances of the diff being emptied. Well the diff is at the back of the box sort of so the oil goes forward...and cornering...well maybe sloshing errr......somewhere....
Sounds like a bearing run dry and running free in the race. Wheel bearings normally rumble when they go because they are under pressure and slowly grinding themselves to bits. This sounds like a bearing that is only 'telling tales' when it's not under torque.
Easy to check if its wheel bearings, Jack front of car up one side at a time; apply handbrake; now with engine running gently spin the airbourne wheel using 1@2 gears then dip the clutch and listen for any harsh noises. Dont do this if your gearbox is fitted with LSD. Hope this helps.