If what you're hearing is "trace rattle" as opposed to heavy detonation, the trace rattle can be a timing/fuel/carbon buildup/operating temperature issue. With trace rattle, if you back off the base timing about 2 degrees, it might go totally away, for example. Heavy detonation would take much more timing retard.
I would think that at just 75K miles, there should not be enough wear in the distributor shaft/bushing interface to significantly affect timing. What might be more important is the wear on the breaker cam in the distributor and related lobe-to-lobe differences, which can cause the timing to not be consistent from cylinder to cylinder, but still not enough to cause even trace rattle unless things were at the "build tolerance stack" situation for compression ratio vs. fuel situations.
Remember that all breaker points NEED to be lubricated when they are installed. I've run across modern point sets that seemed to delete the little vial of lube and without the necessary lube for the rubbing block of the points, the rubbing block will wear prematurely and cause the dwell reading to change and with it, the base timing setting. Point lube is still around, though, even if it's in the General Motors Standard Parts Catalog.
The item regarding the vacuum advance unit might be worth checking out too. Usually, though, if they fail, the rubber diaphram will rupture and cause a vacuum leak. If the timing does not advance sufficiently under cruise conditions, fuel economy will be in the dumper pretty quick and throttle response will similarly suffer. Not to mention the vacuum leak upsetting the mixture. A retarded spark can lead to higher operating temperatures too. It's pretty easy to stick a plug in the vacuum line at the vacuum advance unit to see if that makes any difference, just don't forget to pull it out when you determine that's not the problem. Of course, they're pretty simple to replace so if it hasn't been preplaced, go ahead and get that done.
Now, once you get to that point, be sure to check the advance weight springs inside the distributor. Might be that one of the springs has broken and that particular advance weight is slinging out to the full advance position at any speed. That would lead to over-advanced timing for the rpm being run and could well cause the detonation problem.
A dirty carb will usually run rich rather than lean. When the air bleeds get gunked up and partially closed up with deposits, that send that circuit to the rich side of things. Where the lean situation can come in is when hard crystalline deposits might build up in the fuel feel tubes in the venturi cluster. Those can be pretty easily removed with a small twist drill, going just until you "hit brass", but you have to know where to look.
One other thing that might be more difficult to track would be if the harmonic balancer, with the timing mark on its outer band, might have slipped, thereby rendering the timing mark useless.
If you've run some decarboning/fuel system cleaner through the car and run it on the freeway for a couple of hours in cruise mode, done all of the other tune-up checks, checked the thermostat for correct temp operation (using an infrared heat "gun" to check the temp at the thermostat housing), and still have the spark knock, then it might be best to retard the timing just enough to get rid of it and see how much retard it takes to accomplish this goal.
The other thing would be to use some quality premium fuel (92+ pump octane) to baseline your tests. Then add some quality octane booster to the mix and see what happens. Another alternative would be to get 5 gallons of genuine racing fuel to add to one tank of your super unleaded fuel.
Back when the fuel ratings and unleaded fuel came into the mix, I determined that 95 posted pump octane equated to 100 Research Octane rating, which most of the premium fuels (i.e., "Ethyl") used to be at their peak in the middle 1960s. Similarly, the current posted octane rating of 93 should be pretty close to the 97 Research Octane Rating that the original B/RB motors were specified with in 1957. Using a quality brand in current times is more important that in prior decades, I suspect. You can still find 100 Research Octane rating unleaded fuel through Union 76, Sun Oil, Phillips 66, and similar racing fuel vendors.
If your rattle is determined to not be from fuel issues, it could be due to heat situations inside the engine. For example, if the rear core plugs haven't been knocked out and the system given a good cleaning/flushing, localized heating (in spite of a normal temp gauge reading) due to poor coolant circulation could be an issue.
I suspect that only a systematic diagnostic strategy can really find out what the cause is. In the mean time, getting quality fuel into the system and backing off the timing a little might be the best things to do.
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