Is It Spark knock?

343 Engine running smooth til i get on it sounds like a rattling or a pinging? not sure almost like a catalytic converter rattling but the car still has power etc.

Short answer is probably yes. You are also probably getting knock that you may not even be hearing. Cars can still seem to be performing well with detonation, until the pistons break that is!

Have you checked the timing? I would definitely check the overall timing, the initial and also the rate of advance. Use a dial back timing light and you can get at least a general idea.

I would put the best gas you can in, crank the timing back to around 32-34 degrees btdc total(mechanical plus initial advance) and a nice slow rate of advance. You could even disconnect and plug the vacuum advance as well. Then see what you have.

Another question that comes to mind, did it just start doing it? If it came on suddenly then perhaps something is broken. Also, am I understanding that it only does it when you accelerate sharply? If it was loose bolts in the drivetrain then it would probably do it in other circumstances.

Just a few random thoughts!

Chris

May not be "pinging" - also keep in mind unless the car has been worked on lately, meaning engine work - valve seals and so on, you may simply have carbon build up. We used to solve a lot of pinging caused by PRE-IGNITION, which is very very different from the pinging caused by DETONATION in almost every way - except both cause noise - by removing the carbon.
We'd warm the engine up, take the air cleaner off, fill a Coke bottle with water, hold the throttle open to high idle and slowly pour the water down the carb, being careful to not kill it, giving it more gas if necessary.
Then we'd take it out and drive it on the highway, a good run. 
The next step was to find out WHY the carbon build-up. Often, and the 343 was no stranger to this, neither was the 390 for that matter, VALVE SEALS were often the cause - oil got in and build up the compression for one thing, and the carbon would get hot and GLOW, igniting the fuel mix prematurely, so it caused high compression and pre-ignition and hot spots. 
Cleaning the carbon out with the water or one of the products Wynn's used to offer often did the trick, fixing the reason for the carbon was the next step. 
We found a second common reason - the cars didn't often see the highway....... driven in town a lot, never truly warmed up and gotten hot, up to temperature,  and that caused sludge and acids in the pan, carbon in the combustion chamber, pinging, etc.

You may have other things going on - like mentioned I've seen a loose or cracked flex plate or converter sound almost like a "knock"............. there are a lot of things you can do to diagnose - like temporarily setting the timing back to see if it's detonation. Checking compression to see if there's carbon build up, etc. causing pre-ignition ping, you can look at the old spark plugs to get the story on engine condition, too - there are charts out there that will help you read a plug and see if there's detonation, oil consumption, carbon build-up and so on. 


I take it the coasts get really nasty gas - as around here the fuel really burns pretty much the same and I rarely tune differently for the gas unless I want to run "cheaper gas", lower octane. It was COMMON to call for a higher octane fuel in the late 60s - and today typical is 87. 87 may not cut it - 91 is the more common "premium" fuel here - you have to look to find 93 where 93 used to be common. 

If you have detonation, beware - you can't always hear it, the ping of detonation is a sonic wave caused by two flame front colliding in the chamber........ and if that happens, it can blow the unburned layer of mix off the piston and leave it unprotected against the flames. The chamber gases burn at the melting point of aluminum or above and if that unburned layer is blown away by detonation, the flame touches the piston top resulting in nasty stuff happening. 
Pre-ignition means something hot - like a too hot spark plug, glow carbon, etc. ignites the fuel before the spark does, effectively advancing the timing and causing the burning gases to push back against the rising piston - again, doing damage, including mashing bearings, busting stuff, etc.

I bet someone set the timing by "ear".
This is death in an AMC V8

Set the timing with a light to factory settings
then re-adjust the idle.

91-93 grade fuel should do just fine.

Todays gas is rated in a different manner
but the octane rating of 93 grade is better
than leaded premium of the early 1970's.

What is missing is the Tetra Ethyl Lead.

Also (as correctly stated above) todays gas
has issues in older engines due to the added
alcohol.

But factory Timing settings will get you in the right direction.

By the way, TEL is bad for an engine in every way except it
protects the exhaust valve seats. Add to that it's POSION.