Cayenne Knocking low speed

[Maranello55]

Hi Guys

Looking for some thoughts to this, 5 yr old Cayenne S, at low speed and just as you start to accelerate the there is a juddering/knocking coming from what appears to be the center of the car, does not happen at higher speed, would I be right in assuming its likely to be a Prop bearing ? Any ideas on cost to fix if so ?

Thanks for any input.
PS Not my car (Sister's)