[CliffB]

Thanks for your reply Stout72. I talked to the previous owner today and he said not only is a tire out of round (by a lot, and therefore hard to balance) but also several wheels are out of round making balance a challenge. He and a wheel specialist did the best they could with what was on.

I'll be putting on the slotted aluminum 8 lug mags I have on my 72 C20 which I know are balanced. That will eliminate wheel/tire influence. The prior owner (good friend, racer, engineer who really knows details and has a track record of success with cars and trucks) said that the intermittent up and down rumble could also be the resonance of wheel tire balance coming in ano out of perhaps a driveshaft balance issue. If it is still going on with different balanced wheels and tires, I'll change the mounts I have for the transfer case and transmission and if it is still there, consider driveshaft balance.

It's nice to hear from another Northern Californian. If you know anyone looking, I'll be putting my 72 C20 longbed fleetside up for sale soon. It's a three owner original with 225K. Only this 71 K20 could have gotten me to part with it. I've put a lot into it.

Any other thoughts on the rumble / resonance guys?