Couple of questions

Couple of questions about my 1990 Cherokee Laredo, 4.0 with just over 100k.

question 1 has to do with some top end noise, which I have narrowed down to (what I believe to be) lifter or rocker noise. It is DEFINATELY not bottom end noise, although it took a few nervous minutes with a stethescope to narrow that down. I need help identifying which one it is. The sound is intermittent and sometimes comes and goes, driving in town and sometimes at highway speeds. at highway speeds, I can *just* hear it, then it stops as quickly as it started. It only lasts for 5-10 seconds at the most. With the engine running at idle and with the crankcase cap open, I can see that the noise is uniform with the rockers moving. Is this lifter or rocker noise? I have tried MMO with no success, and just changed the oil with not much change either. The noise is not constant, it comes and goes for a few seconds at a time. Other possibilities that I have not ruled out, but am not sure how to check are:
-fan clutch
-a/c clutch or whatever it is(noticed today that with the a/c on the noise isn't neccesarily present, but that the a/c makes some more grumbly noises)
-water pump
-timing chain

The second has to do with the closed cooling system. My coolant bottle developed a small crack at the top, beside the removable cap the other day. Since I don't drive the jeep more than 10km's at a time, I havn't gotten around to doing anything about it yet. it doesn't overheat, but I'm wondering if it's something I need to replace asap or if I can keep driving it for a while before I shell out some more bucks for an overpriced plastic milk jug.

any insight would be great!

take your valve cover off and see if you have side to side play in your rockers...thats what i have and its an annoying sound...comes and goes....

The valve noise is pretty normal for a 4.0. I wouldn't worry about it unless it gets really loud or other problems come up. Incidentally, the infamous header cracks can make a very similar noise. That bottle on the cooling system should be repaired soon. If you want to put some time into it you could get a fiberglass repair kit from an auto parts store and fiberglass the entire outside of the bottle. That should be a fairly permanent repair.

The worst case scenario would be collapsing lifters. That out of the way check the rockers themselves. BTW while you have the rocker cover off you may as well clean the breathers at either end. Not a problem but consider it preventative. Use a new cork cover gasket not just silicon sealant.
As for the bottle, replace it, and keep and repair the old one for a trail spare. Somewhere on here or another forum I read where someone used a Mr Gasket aluminum overflow bottle as a replacement for the cracked plastic bottle and now have a permanent cure. Got it from Jegs or Summit.
Hope this helps.

Ive probably put over 5000km's on the engine with the ticking lifter noise, but now it's getting a little more pronounced. I'm going to try a quart of risolene to see if that cures it, if not the valve cover is coming off this weekend.