"....but with an overhead cam there are no lifters.." The rocker arms sit on 'lifters' (MB calls them hydraulic compensators or something like that), and if one has failed to pump up with oil, that rocker will make a ticking noise.
I've had rockers wear so badly that the camshaft dug a trench into the facing that the lobe rides on, making a tick each time the lobe enters and exits the valley in the rocker face.
These are super easy to service, though, and as long as the camshaft lobe isn't worn too badly, you can replace hydraulic 'lifter' and rocker arm in under an hour, especially with the right tools (time includes removing valve cover, reassembly, etc.).
Sometimes, if the parts arn't worn badly, a good dose of cleaner/additive will flush out the 'lifter' and get it to pump up again. I'd try that first.
