First you have to figure out why the cam is moving, or if it is. If you already have a cam button in, and it's touching the front cover, which it should be, that could be what you feel...in that case, nothing is wrong with the cam "walking". On the other hand, if you have either one or more lobes going away, and corresponding lifters, you pretty much are screwed. It doesnt matter how long you have run it, I have seen cams go flat during break-in, only 10 minutes....if it has gone enough for you to hear the clatter, then there is lots of metal floating around in your engine...the oil filter will have caught most of it, but lots still gets in the bearings. I just tore one down in which two cams in a row were lost, and all the bearings were shot, along with the crank shaft...it is scored and has to be re-ground. You can just replace the cam and lifters if you want, and change the oil and filter..you might be OK for awhile, if the bearing wear isnt too bad. Or you may be in for trouble, there is no way to say for sure. It would be a gamble. If you do that, watch your oil pressure carefully. If the bearings are wearing out, the oil pressure will start to decrease as the bearing clearances increase. That will be the first sign. It's a tough call, I know you dont want to tear it down, but that's the only way to be sure of what things look like inside. I doubt that not starting the engine for some time is what caused this. It must have been going on since the rebuild. I had one go after about a year once. It happens. What kind of cam is it? Brand name, I mean. What Dr.Bowtie says is pretty much true, I have a roller cam now because of a couple of failures. If you do decide to go that route, let me know and I'll save you some money before you buy anything. Good luck, and whatever you decide to do, keep us posted so we can help. John