Help, Driveline or ? Vibration 71 K20

[CliffB]

I picked up a 71 K20 with 109K original miles yesterday to replace my 72 C20. It drives beautifully. But...

There is a slight rumble on the highway that increases and decreases in amplitude over about a one to two second interval. It is the slow up and down nature of the rumble/noise that makes me ask for help. It makes me think of exhaust resonance or possible non-driveline source. I'm hearing and feeling this at an indicated 65 to 80 mph.

To me, as it isn't constant, that rules out tires/wheels and balance. Does it rule out driveline also? Prior owner did say that a tire measured out of round.

The rumble is relatively minor, but obvious to the ears and you can feel it some.

There is a really strong 1900 mile rebuilt 4 bolt 350 in it. It's a Turbo350. It has a new dual exhaust system from new jet hot coated Hooker headers back with crossover pipe and 2 chamber Flowmasters.

I'm used to 2 chamber Flowmaster sounds as I have them on my 72 C20 and 94 Roadmaster LT1 Estate Wagon.

Any thoughts? Thanks for reading. This is a great site. I'm really glad I found it.