While ferreting about under my engine trying to find the source of a squeak (related to engine movement) that's eluded me for weeks I found the start of a potential problem
On the front of the steering rack there's a heat shield designed to protect the hydraulic pipes from the exhaust manifold. (Applies only to RHD cars BTW). There's not much space there and I found that the heat shield had been rubbing on the exhaust manifold. Eureka!, I thought, that's the source of the squeak - but more of this anon!. However, I also found that the heat shield had been rubbing on one of the hydraulic pipes feeding the rack. Had I not intervened the pipe would have eventually burst.
Needless to say I fixed the problem - by cutting a bit of the heat shield away to clear the pipe and by reshaping the heat shield around the manifold.
As to the annoying noise - well, it disappeared but for only 1 day
Further investigations made me think that the bonnet was to blame - I lubricated all the pivots on the bonnet hinges and stuck some rubber padding on bits where the bonnet had been rubbing on the plastic bits in the engine bay and the heater intake.
Touch wood, fingers, arms toes and legs crossed, I have not heard the noise for two days