Clattering noise from engine bay

[1falconer1]

1999 Au with 170k on the donk.
When starting from cold there is sometimes a clattering from under the bonnet. Sounds loud from inside the car but is barely noticeable outside or from under the bonnet. The noise lasts for about 20 seconds or so and peters out. It is definately engine related as it changes pitch when the engine speeds up when the box is in neutral. Happens only in the mornings when the temperature is lower and the lower the temperature the longer the noise lasts.

Belt?

Any ideas?