Ok I am racking my brains on this:

Engine is a B20B, rebuilt about 25K miles ago. Still using B18 distributor, no vacuum advance.

All this time that I've owned this car and driven it the last 12K miles, I thought the noise I was hearing was caused from my known broken generator bracket. When I installed the SwEm Alternator kit at my parent's property about a month ago, I had to use a lot of washers to get me home because my original bolts were too long into the block. When the sound eventually came back on my 2.5hr drive back home, I simply just thought my washer idea didn't work and that the bolts had loosened. I stopped when I heard the rattle and verified my bolts had not loosened, however I was not too convinced until I got home, cut the bolts, then threadlocked them. Started the car up, no knock. But then after the car warmed up, the knock came back. I checked the bolts again, bolts still tight.

So I hear this slight knock at idle, but realized it's only after it's warmed up, when I rev it up the sound seems to go away but I can't be totally sure. It's not evident though when it's cold. I've checked the timing. It was off just a little. I adjusted it to spec. The slight knock is still there. Checked spark at coil. We're good there. Checked spark at plugs. We're good there. For goods measure, just replaced them all along with the proper gap.

The sound has no repetitive rhythmic pattern to it though as a rod bearing or wrist pin would usually make... It's just irregular.

Also, yesterday, while shut down and battery disconnected, I reached down and grabbed the fan and turned slightly one way, then the other, which of course made the crankshaft turn either way. With each turn in either direction, a hair one way, I hear the sound, then a hair turned the other way I hear the sound... I simply thought the crankshaft pulley was loose and just knocking on the woodruff key, but, the sound seems to be coming from the rear of the block. I laid down under the car, and did the same thing, I turned the crank one way then the other and still swear I hear the sound coming from the rear. Back up top with my ear near the rear I did it again... I also peered down near the engagement arm on the clutch, and see it budge a little up and down with each turn one way then the other.

She seems to run fine otherwise. Smooth idle. Keeps cool, doesn't overheat.

Does anyone have any idea what I could be facing?