603/Britannia ZF Steering Box Strange Noises

[herrbartlet]

Recently bought, but off the road for 4 years, I took the car for its 1st MOT under my ownership yesterday (only 8m each way) and noticed a loud ticking noise coming from the driver’s side of the engine bay. Initially I thought it might have been the hydraulic lifters playing up after a long period of inactivity, but today on another short run, I noticed the noise (which is similar to the cch-cch-cch of a steam locomotive) only appears whilst applying left turn on the steering wheel. I am now thinking the source of the noise is the ZF steering box. Once home, I jacked up the front and with the engine running, turned the steering full lock, both ways, several times, but could not replicate the noise, presumably because the system was not under load. Any ideas??