Kinda hard to tell what that could be but an old trick I would do first would be the screwdriver test.
Take a long handle screwdriver and put the handle up to your ear and put the screwdriver part on top of the valve cover (where the roller rockers would be) and listen. Start at one end and make your way slowly down to the other end.
You'll get an idea of what the roller rockers sound like. If you come across a sound thats doesnt sound right, your problem might be there.
Be sure to check all the hoses, emission plumbing, etc...
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