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driveline hum
My midstates cobra utilizes a Ford 4 speed toploader transmission and a Ford 9" rearend. I notice a harmonic hum
at relatively low engine rpms (1500 - 2000) when the throttle is slightly depressed, like when you are easily cruising on level road. As soon as you let off the throttle completely or increase the throttle the hum dissappears. It seems to depend on throttle position and the lack of any real load on the drivetrain by the engine or the road. Any suggestions on correcting this hum would be greatly appreciated. Thanks Steve |
Will it hum with the car out of gear in the driveway or will it hum when you push in the clutch at the speed when you hear it? Does it matter what gear you are in? Are you sure it is not the tires?
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Can you "feel" the hum, or just hear it? If you could SAFELY put the rear end on jackstands, and "drive " it, would you notice the hum?
Maybe start by checking and tightening every nut and bolt on the car. |
Mine did that. Turns out the driveshaft was too short.
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Since it is only heard at lower engine speeds you should speed up. This also has a tendency to drown out the humming from the happy driver and passenger. Sorry, I couldn't resist.:p
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The hum can only be heard, not felt. If I push in the clutch the hum dissappears. Car needs to be cruising with light throttle to hear hum. Seems as if it is a backlash noise amplified by the driveline or body.
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How hard are you pushing the clutch before the noise goes away? Are you just touching it so the throwout bearing makes contact or pushing it to the floor?
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Bruce,
The noise is still there when the throw out bearing touches the pressure plate, it dissappears when the clutch disk dissengages the flywheel. Thanks for your help, Steve |
Steve,
Sorry one more question. Does it do it in every gear? If it goes away when you take the load off the trans and drivetrain by pushing in the clutch it shows that it is not an engine related sound. So what I am trying to help you find out is if it is the trans or diff. If it does it in every gear it is dif or driveshaft related. If it only does it in one gear it is the trans. |
I only notice the noise in 3rd and 4th gears, however I spend very little cruising time in 1st and 2nd especially with light throttle.
Thanks, Steve |
Sounds like it could be the front bearing in the trans or a gear hum from to much play in the gear box. What type of gear oil do you have in it?
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Bruce,
The transmission was installed new and only has 6700 miles on it. The transmission oil is 80W-90, I do not know the brand. Do you have any experience with driveshaft alignment? I understand you can pickup a harmonic vibration if the driveshaft is in perfect alignment with the rear axle flange and the alignment should be off center a few degrees to keep this from happening. Thanks, Steve |
It should have a slight down angle when the car is flat to the ground. If the pinion angle is off it can cause a problem but I have not heard one hum because of that.
You may try an old trick and drop in a can of STP to see what happens. It will thicken the oil and if it is in the trans help knock down the sound. Of course this will not fix the problem but it will help you locate it. Are you sure it is not rear gear hum you are hearing? Did you have the rear end redone with a new set of gears and bearings? |
Bruce,
This weekend I'm going to double check my driveshaft alignment then try your STP trick to try to identify if the sound is coming from the transmission or the rear end. I was told the rear end was rebuilt when originally installed in the car 6700 miles ago but I am not 100% sure of the extent of their rebuild. Thanks for all your suggestions. Steve |
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