Chasing Vibrations - ‘78 K10 Suburban

[RyanAK]

Hey gang. Working through my ‘78 Suburban drivetrain/suspension to try to get it as smooth and reliable as possible. I have one at 45-55mph that is especially irksome since most of the roads around here are 45-55mph speed limit. Heh.

So this is a cabin boom/drone/buffering at 45-55mph. It isn’t throttle/load dependent - it does it while coasting and in neutral. This indicates the vibratrion is not engine side. It’s low frequency... almost pressure-like. The vibration pulses slightly when it first starts, but by 50mph it’s a constant drone. Once you push through 55, it disappears.

Truck has a new crate 350, TH350 with full-time NP203. Stock suspension.

(Secondary question... what’s reasonable highway speeds with the NP203? Any reason I can’t drive 70 on the interstate?)

I can rotate the rear driveshaft 1/8 rotation while in park.

I replaced the rear U-joints. I replaced the front driveshaft u-joint, but haven’t done anything with the double cardan. Shafts went back in the way they came out. No evidence either have thrown a weight.

There’s occasionally a slight clunk when shifting from park to reverse. This isn’t consistent though.

Tires are 12 year old Toyo Open Country 235/75R15s on stock steel 15x6 wheels. I’m getting a plan to replace these. They’ve been balanced, but I’m not convinced they don’t have other problems... like not being round... that is contributing some of the problem.

Weather seals are ok. No serious water leaks, and wind noise is tolerable. They’re on my short-term list.

This thing is like a huge speaker cabinet... so whatever is going on is being amplified by the acres of sheet metal and huge cubic volume of air.

I have some suspicions, but thought I’d ping the forum for thoughts before I start spending what could be significant time and money chasing this. Other than this, truck is pretty great.

Thanks in advance!



Ryan