Rear (?) Noise ID help
09-06-2014, 08:34 AM
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Rear (?) Noise ID help
This is another sound ID question. Looking for ID or just ideas of what else to check.
A subtle noise. But consistent. I have a couple recordings at the end.
Regularly repeating sound that seems to be at 4 times wheel rpm. Since final drive is 4.10, the rate would at least suggest the things that turn at that speed: differential pinion shaft, propeller shaft, and transmission output shaft. But there is no guarantee that they would make a sound exactly once per revolution, so this may be a red herring.
Sound is only noticeable between 15 and 5 mph, on quiet roads. More noticeable in neutral with engine at idle, which reduces competing noise. e.g coasting up to a stop light. But it is sometimes discernible in gear at low speed. From inside the car, it seems to be from behind.
After recording it from inside the car, I put the car on stands, then recorded using a hand held recorder on a pole reaching under the car near various components, in the cabin, in the trunk, etc. First tried turning at about 10 mph (4th gear at idle). Then to reduce competing noise, I had my wife start it spinning at 10 mph, then push the gear lever into neutral. It slows in only a couple seconds. This got the cleanest recording result. (The tires were on. I found with tires off, without their inertia in the system, the transmission gear clatter was too loud.)
Results: With the recorder next to components under the car I got almost nothing of the sound in question. Certainly nothing to suggest a point of origin. Transmission recorded gear clatter. Prop shaft front and rear joints got normal whirr sound of a rotating shaft. At rear and under differential nothing unusual. Nothing notable near rear wheel hubs, outside or inside or along axles. The loudest place I could find was inside the empty spare tire well. Somewhat less at front of truck well, with tool kit removed but carpet in place. However under the car, between the front of the trunk well and the differential, inches away from the previous location, nothing. Inside the car I can record the sound with the recorder in my hand, but get a better recording with it resting on the console pad just in front of the glove box. I can get at least some near almost any main body structure, for example in the passenger foot-well next to transmission tunnel.
So it seems as though the sound comes from the body of the car. Or rather it is transmitted by the body of the car from wherever it originates, and possibly amplified by flat sheet metal like the floor of the empty spare tire well.
Other info. The axles are new. When I replaced them, I didn't disassemble the wheel hubs, but I did check the wheel bearings for any play, roughness or noise and found none. When I had the car on stands to record, I also removed the prop shaft to inspect its cv joints, and hand turned and tested for any play in the transmission output shaft and the differential pinion shaft. Nothing obvious. After I reinstalled the prop shaft it passed the runout specification. At no time did I hear anything that sounded like a bad bearing.
I live in the country. One or more field mice have stripped all the fabric insulation from the trunk area to build nests. This is the yellow-green material that looks somewhat like coarse felt, glued to the inside of body panels. Other areas, like the trunk well under the carpet, have a painted-on rubbery material that is still intact. The absence of the insulation may contribute to the sound volume.
Recordings follow. The first was recorded inside the car with the recorder resting on the console, coasting in neutral, speed dropping from 9 down to about 5 mph. The second was recorded with the car on stands initially in 4th gear at idle at about 10 mph indicated, then lever pushed into neutral. At that point it quickly slows.
They were amplified in post processing by 20db.
[attachment=65957
n the road coasting 8 down to 5 appx.wav]
[attachment=65958:in trunk on stands kick to neutral.wav]
A subtle noise. But consistent. I have a couple recordings at the end.
Regularly repeating sound that seems to be at 4 times wheel rpm. Since final drive is 4.10, the rate would at least suggest the things that turn at that speed: differential pinion shaft, propeller shaft, and transmission output shaft. But there is no guarantee that they would make a sound exactly once per revolution, so this may be a red herring.
Sound is only noticeable between 15 and 5 mph, on quiet roads. More noticeable in neutral with engine at idle, which reduces competing noise. e.g coasting up to a stop light. But it is sometimes discernible in gear at low speed. From inside the car, it seems to be from behind.
After recording it from inside the car, I put the car on stands, then recorded using a hand held recorder on a pole reaching under the car near various components, in the cabin, in the trunk, etc. First tried turning at about 10 mph (4th gear at idle). Then to reduce competing noise, I had my wife start it spinning at 10 mph, then push the gear lever into neutral. It slows in only a couple seconds. This got the cleanest recording result. (The tires were on. I found with tires off, without their inertia in the system, the transmission gear clatter was too loud.)
Results: With the recorder next to components under the car I got almost nothing of the sound in question. Certainly nothing to suggest a point of origin. Transmission recorded gear clatter. Prop shaft front and rear joints got normal whirr sound of a rotating shaft. At rear and under differential nothing unusual. Nothing notable near rear wheel hubs, outside or inside or along axles. The loudest place I could find was inside the empty spare tire well. Somewhat less at front of truck well, with tool kit removed but carpet in place. However under the car, between the front of the trunk well and the differential, inches away from the previous location, nothing. Inside the car I can record the sound with the recorder in my hand, but get a better recording with it resting on the console pad just in front of the glove box. I can get at least some near almost any main body structure, for example in the passenger foot-well next to transmission tunnel.
So it seems as though the sound comes from the body of the car. Or rather it is transmitted by the body of the car from wherever it originates, and possibly amplified by flat sheet metal like the floor of the empty spare tire well.
Other info. The axles are new. When I replaced them, I didn't disassemble the wheel hubs, but I did check the wheel bearings for any play, roughness or noise and found none. When I had the car on stands to record, I also removed the prop shaft to inspect its cv joints, and hand turned and tested for any play in the transmission output shaft and the differential pinion shaft. Nothing obvious. After I reinstalled the prop shaft it passed the runout specification. At no time did I hear anything that sounded like a bad bearing.
I live in the country. One or more field mice have stripped all the fabric insulation from the trunk area to build nests. This is the yellow-green material that looks somewhat like coarse felt, glued to the inside of body panels. Other areas, like the trunk well under the carpet, have a painted-on rubbery material that is still intact. The absence of the insulation may contribute to the sound volume.
Recordings follow. The first was recorded inside the car with the recorder resting on the console, coasting in neutral, speed dropping from 9 down to about 5 mph. The second was recorded with the car on stands initially in 4th gear at idle at about 10 mph indicated, then lever pushed into neutral. At that point it quickly slows.
They were amplified in post processing by 20db.
[attachment=65957
n the road coasting 8 down to 5 appx.wav][attachment=65958:in trunk on stands kick to neutral.wav]
09-08-2014, 03:40 PM
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Since I posted this on the weekend I thought I'd bump it up for the workweek crowd.
Also I forgot to say that I put the car on stands to change trans and diff oil, then decided to go ahead and do the noise tests first. The old oil that came out was clean and no obvious metal particles. If there was any metal it was microscopic. A very small amount of fine grayish film floated on the surface after I drained one of them. Not "sludge. Just as thin as the old oil. But I forgot to write it down. Diff I think, because could see it as it drained. I couldn't see old trans oil until I pulled the catch container out from under the car.
Changing the oil had zero effect on the noise.
Also I forgot to say that I put the car on stands to change trans and diff oil, then decided to go ahead and do the noise tests first. The old oil that came out was clean and no obvious metal particles. If there was any metal it was microscopic. A very small amount of fine grayish film floated on the surface after I drained one of them. Not "sludge. Just as thin as the old oil. But I forgot to write it down. Diff I think, because could see it as it drained. I couldn't see old trans oil until I pulled the catch container out from under the car.
Changing the oil had zero effect on the noise.
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