Does the oil pressure lamp go off straight away?

Could be a bit of bottom end bearing wear or a noisy hydraulic tappet.

Has it had an oil change using a decent oil filter recently? Poor quality filters don't have an anti drain valve allowing the filter to empty while the engines off.

Sounds like a fault with the VVT system to me. These systems rely on oil pressure to rotate the cams relative to the cam sproket, and when worn the oil drains from them once the engine has stopped. They then rattle on startup until the fill up with oil again.

It will probably be fine for a while, but it would be a good idea to get it changed in the near future because if it wears to the point of rattling all of the time you will loose your variable cam timing.

Unfortunately it is a cambelt off job to change it.