Check really good for an exhaust leak first. Much easier than trying to get off a cam cover. You said it only does it some times...does it seem to be when the motor is hot or cold? Or both. A small exausht leak can sound like you describe and can be very hard to find. Not to mention once the pipes heat up, they can expand just enough to make the leak stop.
As for the rollers and lifters go, they are hydrolic, so nothing to tighen down. You could have a lifter that isn't pumping up possibly, but don't think the sound would go away. Like susie said, get the stethascope out and try to pin point the sound.