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.

[Stout72]

Sounds like what my truck is doing. Others on this Board recommended checking the tire balance, the u-joints or driveline angle (neither of which I haven't been able to get to yet, shame on me!)

Welcome to the site from a fellow NorCal!

[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?

[JIMs70GMC]

CliffB, It could also be a "radial" vibration as in the tires got swapped from side to side Had it happen on another truck and swapped the front tires left to right and the vibration went away. As for your resonance that would be as the vibration from the tires come in and out of phase with each other, I'm experiencing a little of that with my super swampers.

[Stout72]

I have the same mags as you do, CliffB. Just had them shod with new 35" BFG ATKO's by Costco in February. I was planning on getting them rotated anyway (nice that Costco does this for free), so thanks for the thought, JIMs, I'll do this sooner than later and see if the "thrum" goes away. Should do the free, easy stuff first anyway, right? Wouldn't hurt to have them spun as well, in case I threw a weight.

[CliffB]

Is it common to get resonance and varying rumble or vibration with large knobby tires?

I've always used highway or all season type tread and have not had oversize tires. The 71 tires are relatively knobby in design, although not oversized.

Thanks for your post JIMs. It's interesting to know that moving from side to side can change resonance.

Thanks Stout for your last message. I'm looking forward to a major polish job on the aluminum mags. I also received 4 factory 8 lug steel wheels and trim rings (I think 73 to 80s style, grey/silver paint) with the 71 K20. I am not sure yet which wheels to sell the 72 C20 with. I hear people look for the 71 factory steel wheels that came on tthe 71. I'll bet I'll like the look of the later rims with trim rings better for a second set myself.

Merry Christmas you all.

[Stout72]

I didn't have the rumble/vibration when I had the old, worn out 35" BFG MTs on. Rode a lot rougher, too. I have only about 2500 highway miles on the ATs now, if that. Food for thought.

Mr. #1 son owes me a polish job on the mags 'cause I whupped him in a 9-hole round of golf, and he hasn't paid up yet.

Merry Christmas back atcha!