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Foxhallr · 05-04-2018, 09:21 PM

Picked up a new to me 45’ 2008 Magna with ISX 600HP and 75,000 miles in FL and drove it home to TX. Around day 2 my ears adjusted to the coach and I noticed that at certain throttle positions while cruising, a noise and vibration from the rear became obvious. Conditions are easily duplicated to induce it: any cruise speed between 40 and 65, slightly elevated engine rpm but low torque...as in cruise control attaining target speed and backing off slightly. Totally absent under load and while coasting. I can induce it with accelerator by changing throttle position slightly while in cruise but not under load. Wife says it’s really loud under rear floor. Didn’t think to try different gears but will. Seems most correlated to engine rpm and load or lack of. Called PO and he said he’s had this as long as he can remember and many shops have checked it and found nothing. Reading some of the threads here, some mention ride height as a non-intuitive cause of some issues. Suggestions? Heard of this before?

Mr_D · 05-04-2018, 10:58 PM

Ride height wrong causing U-joints to bind or bad U-joints.
Slip joint not lubed.

John Jones · 05-05-2018, 09:17 AM

Quote:

Originally Posted by Mr_D

Ride height wrong causing U-joints to bind or bad U-joints.
Slip joint not lubed.

Exactly what I was thinking. My 2000 Allure had this same problem when I bought it. I raised the rear end to the proper spec and the noise went away.

deandec · 05-05-2018, 11:02 AM

I had the problem with a new set of rear tires. @20 tire dismounts later we changed out the new tires and the vibration disappeared.

Foxhallr · 05-05-2018, 02:19 PM

Thanks. So i’ll Tackle it as a ride height issue first. Now a newbie question: where does a repair shop find the proper ride height spec for this coach and how does it get adjusted? Is this a specialty chassis shop type thing or can any good mechanic do it?

BrianGlenn · 05-06-2018, 09:30 AM

Quote:

Originally Posted by Foxhallr

Thanks. So i’ll Tackle it as a ride height issue first. Now a newbie question: where does a repair shop find the proper ride height spec for this coach and how does it get adjusted? Is this a specialty chassis shop type thing or can any good mechanic do it?

See attached.

Brian

rvbiker45 · 05-06-2018, 10:06 AM

Good suggestions from others about the issue you are having being caused by ride height. Then maybe not. I will relate my noise issue and my cure for it.

85,000 miles on the DSDP with an ISL 400. I had no noise during a walk around while the engine was running. Into a trip about 250 miles, I noticed a noise that I thought was coming from the dash. Nothing I did would make the noise stop or lessen. Upon arrival at my destination, a friend who had been waiting for me said he heard a high frequency noise coming from the MH as I turned off of the highway into his driveway. It is about a 1/4 mile driveway on a farm. During closer inspection, we found the louvered vent for the side mounted radiator was making contact with the frame around it. Thus setting up the harmonic. While going over the possible causes for this to happen, we discovered that the radiator rubber shock mounts were totally flat and resting on the mounting brackets, making metal to metal contact. We changed the rubber mounts, from Spartan, and the noise and vibration did not return. This may not be your issue, but just something to consider

Foxhallr · 05-09-2018, 10:27 PM

Ride height in spec. Lubed U joints and driveshaft appears all tight. Mechanic didn’t see a way to lube slip joint although rubber boot was visible in driveshaft. He was an alignment guy. But, not wanting to make this easy, he found a badly leaking front wheel seal and 2 torque control arm bushings bad. Thanks to Brian’s pdf file for suspensions, I was able to speed up parts identification for the bushings (Ridewell). Sourcing parts now. All this may or may not have anything to do with my original vibration issue. We’ll see.