1985 Nissan Sunrader loud rear noise

[Carlor]

Hey y’all, just bought my first Nissan rig…it’s one of those Sunrader RV’s with the dually rear ends. 
 

I saw it for sale online for a pretty great deal and flew out and bought it and immediately drove it 300 miles to the west coast to keep it at a friends house in Southern California until next month when I plan to drive it back across the country. I live on east coast.

 

When I got it to low highway speeds and up I immediately noticed this super loud low pitched humming/vibration when giving any gas to the pedal. I tried to see anything that affects the sound and this is what I came up with:

 

-Without fail the sound goes away immediately without when taking foot off gas while in gear and immediately comes back when foot is back on pedal. Taking foot of gas in neutral doesn’t do anything, the sound is still there. 

- Sound goes away for a quick second when releasing clutch after changing gear

- The sound is slightly quieter in higher rpm’s

- Can feel vibration from touching any part of car in the cab , but nothing that feels like origin of sound

My thoughts are worst case scenario the rear axle is shot. I’m also thinking about u joints and trans mount. How often do the axles fail? It’s the full float axle. Would u joints or trans mounts be able to be the source of the noise? I mean it’s LOUD.  Am I looking at the wrong thing completely and should be thinking about the trans?
 

If anyone has any experience with any of this and has some suggestions I’d love to hear your thoughts.  I’m trying not to just throw parts at it but I live on east coast and am only going to be able to diagnose/work on it when I get to west coast next month.

[datzenmike]

First check the differential oil level. Should be level with the fill bung on the back.

 

 

Your truck has a two piece drive shaft with 3 U joints. Where the front joins the rear drive shaft there is a yoke and a support to stabilize it. Looks like this...

 

 

Inspect the rubber isolator. If it's damaged the drive shafts will go out of position under load and vibrate. There are 3 U joints. Push and pull on them at right angles to the rotation. There should be zero movement and only turning.

[Carlor]

Thanks for the reply! Hmm I took a quick glance at the center carrier bushing but it looked fine to me. Granted, when I say quick glance I didn’t actually crawl under. I was pretty strapped for time since I had to make my flight back home. I didn’t give it the proper inspection it deserved. Because of this I changed my flight to a couple days earlier to give myself time to work on it before driving back across the country .

 

 

Would the center carrier bearing explain immediate disappearance of noise when I take my foot off the gas while in gear and it coming back when I give it gas again?

[Carlor]

Also to maybe further describe the sound it was making, it sounded like the drone of an airplane engine or like car with the cat cut out. It wasn’t shuttering or terribly shaking the vehicle.

[datzenmike]

1 hour ago, Carlor said:

 

 

 

Would the center carrier bearing explain immediate disappearance of noise when I take my foot off the gas while in gear and it coming back when I give it gas again?

 

Not the actual bearing but the rotten away rubber surround will allow the driveshaft to hum loudly under load. If the rubber look alright then it's fine...

 

 

 

Could be a bad U joint that has a lot of play. Under load the driveshaft goes off center and THAT will vibrate badly.

 

Look for broken leaf springs, loose U bolts, loose shocks and yes loose transmission to engine bolts and engine and transmission rubber mounts.

 

Swap the tires from the back to the front to eliminate them as the cause. Loose lug nuts???

[Carlor]

All good ideas, thanks. Do you know if  any of these parts are specific to the sunrader/heavy duty truck? Since it’s a longer frame (I think?) are the ujoints/ any other parts different? It’s been a little tough differentiating parts.

Also I remember reading somewhere the rear wheels are different sizes than the front? 
 

 

[datzenmike]

The company that buys the truck buys what is called a Cab/Chassis option. It a purpose built commercial use truck that arrives without the box. It's assumed a camper or work box will be installed. The '85 will have long wheelbase/regular cab with dual wheels, a different rear axle, 5 speed standard transmission,(no automatics) HD radiator and a few other things.

 

 

Rear rims may be different from the fronts but all tires are E78/14

 

U joints and front and rear brakes should be the same as the 2wd truck.

[Carlor]

Awesome, thanks datzenmike! That’s all super helpful. 

 

I wish took the time to take pics of it to show y’all. I’ll have some in a few weeks.

 

I’ll update with the work I do on it then. 

[bottomwatcher]

Since you have a little time before the big trip you might want to gather a couple extra parts. 2 fuel filters 1 to change before the trip and a spare for on the road. New belts if you don't change at least have on hand. A waterpump, these trucks are getting old and water pumps are not always in stock anymore. I have driven my 720 across the country multiple times and have had to change waterpump on the way a couple of times. They do give a warning by leaking out of the weephole which is fine when you only drive short distances but on the road the difference between weephole leak and full blowout is only a few hundred miles. Buy a quality one and change before you leave or buy a cheap one and keep as a spare. For me I would throw on a quality one at the beginning of the trip they are a wear item. Just my 2 cents.

[Carlor]

Well the noise turned out to just be the rear axle u joint. Replaced the center carrier bearing as well. Sound completely gone!