So, I just completed my first successful engine swap from my blown '75 1.8 to a new used '72 1.7. However, after firing up the engine, there is a strange clattering sound. The valves are adjusted perfectly, the distributor, wires, plugs, and timing all check out correctly. Compression also checks out fine. It doesn't sound like a thrown rod or a bent pushrod. I also don't think it could be a foreign object in the case or cylinder, as the sound is only present when at low RPM. One revved, the sound goes away (as opposed to getting louder). My friend thinks it might be the carbs (dual Dellortos) running rich, and my dad speculates the oil might not be circulating properly, because the oil I drained from this new (used) engine before putting new oil in was pretty black and could be clogging up the oil screen. Any thoughts?

I would try to isolate the sound first. Get one of those stethiscope (sp?) thingies and see if you can trace the sound to the source.

Maybe it;s something simple like loose tin or something in the fan shroud.

It is so hard to make any diagnosis without being there. I remember having a clattering noise with my 2.0. Later I lost compression on 1 cylinder. Problem was a loose valve seat. The seat was so badly worn out, I could hardly believe the engine even ran. Compression was a little low on that cylinder. In my case the compression test was not enough to tell me that I had such a serious problem. On the other hand your problem may be something really simple like a loose fan belt. Lots of luck on your problem. I hope it is nothing too serious.