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I'd start by checking all the exhaust joints - where the manifold joins the head and the down pipe joints just before the catylst - the best way to do this is to start the engine from cold and feel around each joint with your bare hands - feeling for exhuast gas blowing over your fingers - you'll have about 30 seconds before it all gets too hot.
if this is OK then I'd next try some oil flush treatment, as you've already found the cam phaser / actuator can stick
if you still have the noise after the oil flush and fresh oil
then yes I'd look into replacing the cam actuator parts