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My Daihatsu Sirion (2006) with 92000k mileage has a light engine knock/grumble when starting from cold. It is minor and intermittent and lasts for about 30 seconds after 1-2 minutes running under light load. Worse when air temperature is colder. Noise disappears quickly and does not reappear until vehicle has been standing overnight. Does not burn oil and oil does not get very discoloured between services.
Vehicle has been serviced by Toyota at recommended intervals since new. Toyota used slightly higher viscosity oil at last service, but otherwise aren't much help.
Causes might be a lazy oil pump, or a sticky oil pressure relief valve. Maybe the camshafts are getting worn and need shimming or timing chain tensioners are wearing. ???
Any advice would be appreciated

Hi there,
A number of factors can contribute to a light rattle when starting cold and may need further investigation.
It does sound like it may be oil pressure related and affects those items relying on it.
Lifters, engine bearings (cam and crank), and cam chain/tensioner. The most likely, or a starting point would be the lifters if they are hydraulic, followed by cam chain and tensioner, as the other option could require an engine overhaul. It may be something that is not economical to actually remedy until a certain fault is able to be pinpointed or it may not even get much worse. But it would take a bit of diagnosing to say for certain what would fix it.

Thanks Cade. Hopefully it's just the lifters. Maybe time for a engine flush ? Although looking into the top end via the oil filler hole reveals only light brown colour and no nasty stuff. Maybe that's why Toyota service just put in different oil and said things should quieten down after about 1000k. Maybe that's why the noise is variable as well.
Apart from this slight annoyance, the car is ideal for our use and spends most of its life running briskly along the Northern motorway. I know the KV3e engine is pretty bullet proof.
Thanks again for your comments