Driveline Hum on Decelleration on Highway - Ford Powerstroke Diesel Forum
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Old 08-14-2013, 05:37 AM
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Driveline Hum on Decelleration on Highway

Hello all- I'm relatively new to this board, as I just bought my 7.3 truck (2000, crew cab, long bed, 4x4, automatic trans, 147,000 miles) about a month ago. A few days ago I took it out on the highway towing my car hauler and two cars, approximately 9000 pounds. I noticed when decending a hill, or just coasting in general, in OD, between about 68 and 63 mph the driveline makes this humming sound that sounds like the truck is running over rumble strips on the side of the road. It doesn't happen at those speeds if I'm accelerating or maintaining speed on a flat or hill. It only happens if I'm doing, say, 70 or so and the truck coasts down. Once speed drops below 62 mph or so the noise stops almost instantly.

If I put the truck in neutral at 68 mph and coast down, I don't hear the noise.

If I leave the truck in gear (OD) while coasting down and lightly press the brake pedal, the trans seems to shift into a coasting mode, the rev's drop to around 1000, and I don't hear the noise.

If I put the transmission in '2' and coast down through the same RPMs as the truck would be in in OD at around 65 mph (About 1800 RPM, and 50 mph) I don't hear the noise.

The transmission seems to shift fine, no weird noises or slips on accelleration, and the engine has good power and seems normal. I checked the trans fluid and it was on the low end of the "Ok" range on the stick, so I added about a pint and a half of fluid to bring it to the full mark. Didn't make a difference.

I made the return trip with an empty trailer, about 3000 pounds, and the noise was the same.

Anyone have any ideas? Thanks!
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Old 08-15-2013, 04:36 AM
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Wow, silence...

I went out and took a look at the truck last night. The transmission was just about sitting on the crossmember. So, I got a new trans mount and put it in- seemed to raise the back of the trans about an inch. I'm wondering if the increased U-joint angle there was causing the noise.

I also read some other threads about driveline noises and topped off the rear diff- it took about a pint of 75W-140 synthetic with the limited slip friction mofier mixed in.
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Old 08-15-2013, 07:24 AM
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i'm fighting what seems to be the same noise, for the last 2yrs , but mines a 6spd. With the amount of time the driveline was being used with the pitched angle, its plausible you killed the u-joints and center support bearing.
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Old 08-16-2013, 04:38 AM
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I drove the truck to the landfill yesterday, and the noise seemed to be gone. *crosses fingers*. I checked the U-joints and center bearing while I was under there, and didn't feel any play or slop. *knock on wood*
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Old 08-19-2013, 05:29 AM
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Nope, noise is back- it seems to be worst when towing a trailer. Just driving the truck around normally, I get a bit of a low-pitched hum on accelleration ~55mph in OD, and under coast in OD between 70 and 60 mph. The noise is much louder when towing, though. Took the truck to the shop today, and am going to have them take a look at it for me.
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Old 08-19-2013, 05:59 AM
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Drop your driveshaft and check for backlash through your transfer case and backlash on your ring gear. Also no play on the pinion as it should have preload.

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Old 08-19-2013, 08:51 AM
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I'm going to agree with the above post, normally a noise on deceleration will be in the ring and pinion...
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Old 08-20-2013, 03:40 AM
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Took the truck to the shop yesterday, they checked out the driveshaft, U-joints, etc. Said everything looked fine. They can't get the noise to happen, since it only happens noticeably when towing and at speeds over 60 mph. They did say it looked like the exhaust was rubbing on a cross-member and could be resonating and causing the noise.
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Old 08-20-2013, 07:16 AM
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I bit off subject, but do you tow a lot? Some will argue, but you may want think about a MagHytec cover or something similar for your diff.

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Old 11-21-2013, 11:09 AM
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Turns out the problem was the trans crossmember was bent- bowed down in the middle. This caused the driveshaft to have the hum to it under certain conditions. I shimmed up the trans end bushing about 3/8" and the noise went away. I'd have preferred to just replace the crossmember but they're like $200 used! Have other people experienced them bending like that? Ick.
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