It probably took you longer to post this question than it would have to check the spark plugs.
If this randomly happened while driving to someone who never pops the hood. We can throw alot of guesses at your problem but since you managed a manifold replacement. You are obviously comfortable under you hood.
You have an exhaust leak. Even the manifold to exhaust can have a ticking sound. Torque the bolts again and listen for a change ticking sound.