Excessive exhaust fumes when reversing

[Guinness]

Hi,

For a while now I have been getting a lot of white exhaust fumes when I reverse. There is nothing when driving normally or when I accelerate.

I have a 96 A4 1.8SE, it recently required an emissions flush in order to pass the MOT which seemed to resolve the issue for a week or so.

Any suggestions of what may be causing this please? Could it be the Cat?