I second the flexplate/torque converter thing. I'd check the bolts first, then look at the flexplate where it bolts to the crank. This can be hard to see anything. I used a little dental mirror and a halogen light, spinning the flexplate with a prybar, and found 2 small cracks coming out from a bolt hole.
This was giving me a clicking noise, but only under light acceleration.
It could very well be what the others have mentioned too.
67 396 race car project
67 SS 396,3-sp mothballed for resto
67 RS 327,4-sp
72 RS 350/350, under current resto
69 4X4 suburban 350,4-sp
73 3/4 ton 454/400
69 Cadillac ElDorado
60 El Camino keeper? project? Yard ornament? stay tuned.