Low Hum on light acceleration around 30-45mph.

[WindHarvester]

Alright,

I've been observing this issue for a little while now and it has me a bit confused (not that hard when it comes to cars) so I'm hoping someone here might have an idea of what could be causing this. I have an AWD 1.8 vibe with an Auto transmission.

I've noticed that when driving around 30-45mph if I am coasting and I lightly press on the has peddle I get a low hum/vibration, feels like it's coming from the transmisson, the best thing I can think to compare it to would be the feeling you get when you shift a manual car in to a gear that is much to high and try to accelerate. As soon as I press harder on the gas peddle the noise goes away (feels like the transmission shifts when this happens).

I usually notice it one to two times on my way to work (City traffic driving for about 7 miles). I went on a 150 mile trip this past weekend and was playing around trying to get it to happen on some country roads and couldn't... maybe something was warmed up enough that it wasn't happening? This morning on the way to work I observed it again though.

I'm hoping this doesn't spell anything majorly wrong with my transmission, though my hunch is that it's related somehow.

I'm planning on dropping the transmission pan and doing a partial drain and refill of the ATF fluid soon... but in the mean time any theories on what could be causing this weird behavior?

Thanks all!

[ehoff121]

Have you checked the transmission fluid for any odd smells, discoloration, metal specs? If it hasn't been changed, clear reddish brown would be OK for old fluid.

This noise could be related to the rear differential.

On a paved road with the interior accessories (A/C blower, radio) off and windows up, listen for the sound. Does it change pitch with speed? Some have described it as a groaning or the sound a manual makes when in reverse. You'd hear it around 15 MPH (low growl) and all the way up to 45MPH, when the pitch get too high and blends in with normal road noise. It would be most noticeable around 25-30 MPH.

Have you checked/changed the transfer case and rear differential fluids?

[Caretaker]

sounds perfectly normal to me. What you are hearing is the car is in overdrive and stuck between gears. You will get a low rumble when the transmission struggles to downshift to a lower gear. You can thank Toyota for the crappy gear ratios on these Vibes. A four speed tranny is so yesterday. Nothing is wrong with the transmission. It just needs to be a 6 speed and not the lousy 4 speed we are stuck with. Since you did not say what year your Vibe is, if it is the first generation, you can use the overdrive off button on the shift column to help the transmission when doing city driving. Oh, and make sure you change out the transmission fluid every 60,000 miles. Toyota Type IV fluid for the first generation Vibe and World Standard fluid for the second generation Vibe. Enjoy.

[WindHarvester]

So I bought the car about 2 months ago, and I checked the ATF Fluid then, didn't smell burnt, no flakes that I could see. was a clear reddish color. I'll check again tonight to make sure nothing has changed. Didn't notice issue this when I first purchased the car, but I remember first hearing it not that long after... so judging from the seemingly intermittent nature of the symptoms it may have existed prior to my purchase.

Rear diff is an interesting possibility, I have not checked/changed the transfer case or rear diff fluid, i feel like that would be a more constant/consistent noise though, as you describe.

The noise only happens at very specific times, it is not constant and does not blend into road noise above certain speeds or anything of that nature. I have to be coasting and then very lightly press on the gas (so lightly that it barely would cause acceleration, more the type of gas pedal press that you make before you actually press on the gas for real) as soon as I press down the rest of the way the noise goes away. It's almost like the transmission is in the wrong gear and then wakes up and realizes that it needs to change gears or something.

[WindHarvester]

sounds perfectly normal to me. What you are hearing is the car is in overdrive and stuck between gears. You will get a low rumble when the transmission struggles to downshift to a lower gear. You can thank Toyota for the crappy gear ratios on these Vibes. A four speed tranny is so yesterday. Nothing is wrong with the transmission. It just needs to be a 6 speed and not the lousy 4 speed we are stuck with. Since you did not say what year your Vibe is, if it is the first generation, you can use the overdrive off button on the shift column to help the transmission when doing city driving. Oh, and make sure you change out the transmission fluid every 60,000 miles. Toyota Type IV fluid for the first generation Vibe and World Standard fluid for the second generation Vibe. Enjoy.

THIS! Thank you, this is what I wanted to hear, and seems to describe exactly what i am experiencing. My Vibe is a 2004, so i do have the overdrive button, I'll give that a shot and see if it makes any difference!

I have only driven 5-speeds and manuals in the past so I kind of had a feeling that this might be related to the 4-speed transmission but I wasn't sure. I'm still a little paranoid about finding spooks after buying this car a few months ago.

[Caretaker]

Well, the Corolla transmission is one of the best ever built. It should not fail on you. I only hate it because I have (at the time) a brand new 2009 and get stuck with the same old, same old............a four speed transmission, when Hyundai is beginning to put 6 speeds into theirs. Also, my 2007 Fit has a 5 speed automatic. So while it won't break down, my anger is that it doesn't have one or two more gears. Definitely use the overdrive off button when going up hills in suburban traffic. Doing so will definitely take some of the strain off of the transmission when you have been going around 45 and then suddenly decelerate to 35 and the transmission is still in O/D. I'm confident this is your issue and you don't have any mechanical troubles whatsoever.

[WindHarvester]

I can totally understand your frustration! I don't mind it that much personally... but I bought this car with the full knowledge that it might not be the "most" fun to drive, more of a workhorse really.

I drove home yesterday, and to work today with overdrive off and did not experience any of the symptoms I described above. I have a mechanical engineering degree, so my mind is very capable of coming up with all kinds of horrible things that can go wrong with my cars... but I have limited actually experience wrenching on the cars to balance that out with practical first hand experience... so sometimes I think I get a bit carried away imagining potential problems.

[ehoff121]

Sorry, I should have focused on the speed and the "light" pedal.

As Caretaker said, that is the low end of overdrive and it will rumble at 35 MPH or so. You should also be below 2k on the tachometer when you hear this noise.

Lower RPM's + higher speed = better fuel economy. The sweet spot is right around 40 MPH. Turning O/D off increases power, but decreases MPG. Some would rather not deal with the noise/vibration, but I leave my O/D on all the time.

If you find the vibration to be too harsh, cleaning the throttle body, new spark plugs, and ensuring you have a good seal on the intake manifold gasket may help.

[WindHarvester]

ehoff121, no worries!

I kind of thought it was something like the overdrive, but I was hoping someone would suggest it without any confirmation bias of my own seeping in. It does happen right about 1700-1900 RPM, so that also fits. The suggestions about throttle body and spark plugs are good, I'll make sure to add that to my list of things for Maintenance! I think I'll probably toy a bit with turning overdrive off and on depending on where I'm driving, as the car feels a bit more responsive and "fun" with it off, and I really don't like the noise when it happens. If my mpg takes a significant hit I'll reconsider.

Thanks to both of you for assuaging my fears!

[ehoff121]

Another easy maintenance item that would help is cleaning the MAF sensor. Just be sure to remove the battery negative cable before doing it.