Seat Ibiza knocking/ grating noise/vibration from front end. not engine problem.

I'm getting an intermittent knocking or grating noise. I know its not the engine because its most evident when I freewheel the car with engine off (in neutral). Frequency of grate or knock related to speed, low speed = low freq. and higher speeds = higher freq.

I've tried to replicate noise with car jacked up on axle stands but then I dont get any noise/vibration. As far as I can see its not the brake disc or wheel bearing which both seem ok. My feeling is something loose or rattling around inside the gearbox casing where driveshafts connect to clutch plate?? I'd like to get a better diagnosis of fault before I get quote for repair.
I do worry I'm going to do further damage by continuing to drive, but sofar the car drives OK apart from the intermittent noise which occurs mostly at low speeds, although I did get some loud knocking while at 70mph on motorway commute, but this was a week ago and hasn't happened since.