Theres a rumbling noise when i let off of the accelerator in my truck?

Kylie

I have a 2004 Chevy Blazer. I recently got the brakes fixed on it, I had a leak in a break line and they went out on me. About 10 minutes down the road from the shop I started hearing a rumbling noise. It started off as a high kinda humming noise and then got rougher and deeper as I went. It happens when I let off of the accelerator until I come to a complete stop. The humming usually starts when I hit around 40 mph or above and the rumbling starts when I let off the gas and just coast until I come to a complete stop. If it’s making the noise and I accelerate it gets quieter and closer to the humming again until I coast and the rumbling returns. https://photos.app.goo.gl/EU6nuksxsHQWwVyd8 The link above is to a video of the noise. It starts about 8 seconds in and gets considerably loud about half way through or so. Let me know if it doesn’t work. If you got any ideas as to what it is I’m all ears, thanks.

PF_Flyer

Just judging by the sound, (and thanks for the video, that will really help others to help) it seems to have nothing to do with the brakes. If it did, I would expect more of a dramatic change as you step on the brake to slow down and stop. I'm thinking just a coincidence that it started after your brake work.

Doesn't sound like any wheel bearing I've had, but it could be I suppose. Since the sound seems to change with the speed, might be related to an axle. And your description of how the sound changes from a hum to a grumble when you let off the gas makes me think something around the axle as well, like an axle bearing or CV joint, and the sound is changing when the load changes on the axle. Let me share some images to explain what I mean.

Yor basic axle assembly


Easiest way to show what I mean about the load changes on the axle was to change all the parts of the CV axle to a chunk of pool noodle (don't ask why I think this way, it just happens )

Simplified axle


When you are accelerating, the engine/transmission is turning the axle forward to drive the wheel, and the forces in the axle want to twist it like this:


So the load on bearings and CV joints is in a specific direction. (my left hand twisting forward, right hand twisting back)

When you take your foot off the gas and the vehicle is slowing down, the forces are in the opposite direction:



So anything in that CV axle assembly that's making noise could be making quite a different noise under the two conditions.

Some of our wiser forums members may now shoot holes in my thoughts

Kylie

Ok so I found this under my blazer in the front? I didn’t notice it there previously but then again I never really looked? That may have been stupid of me but I saw it hanging down on my way back to my truck after stopping at the store. Know what it is? I’ve got zero knowledge with cars so 😬😬. 

PF_Flyer

You also didn't specify, is your Blazer four wheel drive?

PF_Flyer

Not exactly sure (asking some folks) what that is, but I did some playing with the photo to get a better look at those "horns". I suspect those might be protection for the undercarriage (could be wrong as I'm guessing). They certainly look like big chunks of steel protecting the axles from things like rocks someone might encounter offroad.



In any event, they don't look connected to anything that's involved in making your Blazer move, and the noise you describe and have recorded in the video sure sounds like something in the drivetrain.

imidazol97

Kylie said:

Ok so I found this under my blazer in the front? I didn’t notice it there previously but then again I never really looked? That may have been stupid of me but I saw it hanging down on my way back to my truck after stopping at the store. Know what it is? I’ve got zero knowledge with cars so 😬😬. 

That's a sway bar. I can see the eye holes at end end.




The eye holes _used_ to have "links" connecting the sway bar to the moving part of your "A-arms" on the front suspension. The links typically have rubber bushings on each end to soften vibrations and a rod through the middle. The rod corrodes away with salt and beet juice used to treat the roads for winter here in the Ohio area.

Usually one side breaks and gives a rattle noise that folks notice when turning in and out of driveways at lower speeds. But on yours it looks like both broke.

Here's a video that will show replacing them. Can be a do-it-yourself job if you have proper jacks to lift both sides at the same time to avoid any sideways uneven heights--that causes the sway bar not to be in a neutral position.

Good luck with the replacement.
I have not previewed the video. Sometimes they're not useful. I thought the initial picture shows the same type of hefty sway bar that you have hanging down, so I hope it's good to go!

https://www.youtube.com/watch?v=jD0B_mX_vXY

PickFix

anybody still running into this issue should most definitely check out there U-joints on the main drive shaft. slip you vehicle into neutral, apply the parking or emergency brake (or just block the wheels so the car cannot move), crawl underneath your car and test the play from your U-joints to your driveshaft by physically spinning your driveshaft back and forth. The shaft and the U-joint should be moving perfectly in synch with no delay whatsoever. If there is a delay, depending on how bad you may have the effects of a "clunking" noise when shifting into drive or reverse or this rumbling when letting off the accelerator. Be sure to check all of the driveshaft U-joints for a fault as some cars have 3 vs others having 2 on the main driveshaft. If not taken care of in good time, you may find yourself stranded on the side of the road with not only a U-joint to replace, but also an expensive driveshaft. U-joints can be purchased from prices anywhere from $15 to $70 depending on certain brands and warranties. Hope this helped, have a nice day!