Worn rockers will cause the same sound if they have worn to an extent that the hydraulic lifters can't soak up the clearance, very unlikely however IMO.

As Jambo said they don't always quieten down straight away but from personal experience its usually only a matter of getting the car hot and driving it a few miles before the air is gone.

Try taking the rocker cover off, and using the rule of 9, play with each rocker and see if there is any play. If its a problem related to excessive wear it will be evident when doing this.......