This is a new one for me. Back history first: 1979 stock Super Six. Before I put it on the road last month I tore off the valve cover and replaced all the valve stem seals. The inside was a goopy mess. I cleaned the rocker shaft out and soaked the rockers in a can of carb cleaner overnight and blew out all the oil passages till they were clean. Cleaned the top of the head spotless and powerwashed the inside of the valve cover. Set the valves cold at a loose .010 and .020. Replaced the wires,plugs, cap and rotor. Rebuilt the BBD. Set the timing at 8 BTDC. Calls for 12 BTDC under the hood. Car runs like a top. couldnt ask for better. Gets around 22 m.p.g average combination city/highway driving as everyday transportation.
Now for the noise its a weird one. Sounds like rattlin valves to me but it does it when i'm at partial throttle at cruising speed. Let off the gas pedal it stops, push the throttle a little more down just enough to get it to ever so slightly to speed up and it quits. At the slightest pedal once your at speed it doesn't do it either just when the pedal would be depressed about lets say 1/2" to 3/4". Less pedal or more pedal it stops. And if you accelerate hard and let off the pedal it will rattle a half a second as the rpm's come back down.
OK so today I put a remanufactured distributor in it thinking maybe the timing was flying out to quick. Because I noticed under the timing light it looked like the mark was bouncing about a degree plus and minus past the set mark. New distributor it did stay pretty stable less than a half degree of flutter at idle. Set it at 10 degrees BTDC. It was about 40 degree total mechanical and vacuum at 3000 rpm. Test drove it and no change. Dang it! Anybody else ever encounter this one?

Thought I'd add this. The rattlin sound sounds just like a diesel engine especially when you let off the throttle after accelerating.

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I don't know a lot about it but 40* of total advance sounds like a little much to me. I would think about 35 is more realistic. The noise you're describing does sound like detonation. I'd back the initial timing down a little to about 5* and see if that doesn't help it. Also check for vacuum leaks. Another possibility is the heat riser valve rattling.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

I'd bet that isn't helping much,...those valves should be set at operating temp! Everything gets tighter when it heats up, and if those valves are staying open.....(blowtorch effect)
Hope you get it fixed easily!
BBBanjo

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225 Cubic Inches of Iron-Head American Muscle

225 bored .040 /.100 off block, Schneider Cam 224@.050~ .480 lift - Stock valves, blended bowls, Offenhauser intake with 500 Edelbrock carb

Sounds like it's pinging with too much vacuum advance.......

Go for a ride, make sure it's doing it, then pull over and plug/disconnect the vacuum advance.

See it still makes the noise.

If it doesn't, adjust your vacuum advance can so it takes more vacuum to advance the timing.


If it does................. don't have a clue at the moment

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

A PCV valve can also make a "rattle" noise under certain conditions.
Disconnect or swap-in a different one to test.
DD