If this sound occurs while running;
you got wires crossed? 18436572. #1 is driver's side front, and the cylinders are numbered from front to back.
or bad timing, as in pick-up polarity reversed? Check your strobe-action; it should move smoothly with rpm, with no jumping around, no missed strobes and no extra strobes. If this happens, just reverse the polarity and reset your base-timing.
or is your idle-timing excessively high?
You got an MSD? then you need a compatible lite.
If that sound only happens when you shut her down, and the engine fails to shut right off, then it is dieseling. The throttles are too far open.
I never adjust my idle for highest vacuum. I set the rpm to what the engine wants based on;
1) the lack of tip-in sag, and
2) not banging into gear, and
3) a modest rpm drop when going from N/P to D.
Then the mixture screws should be in the center of their range;
~.5 to .75 turn for Holley-types and
1.5 to 2.5 on Carter Types.
If they are not, I can usually trace that to the transfer slots not being set right. I leave the vacuum gauge for other stuff.