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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!