Another item to check is the air injection pump, which runs for about 3 minutes at startup. I had a persistent ticking as well and when my mechanic disconnected the pump the ticking went away!
Left it disconnected (causes no running problems but may violate air quality laws - check local statutes) but after a few weeks the check engine light went on cause it was registering as a fault. Reconnected the pump and put up with the noise for the first few minutes every morning.
After about a year the ticking noise went away....go figure?
"Lead, follow, or better yet, get out of the way!"E500 owners motto