Just another wierd vibration question

john-ee · Oct 8th, 2005, 02:01 AM

Hello everyone, I'm John and I'm new to the forum. Hope someone can help with this as it's driving my wife and I nuts.

VEHICLE:
1999 Nissan Sentra GXE 1.6L Automatic 159k mi
Engine very well-maintained. Purrs.
4 new (~5-6k mi) Dunlop SP40 tires on the alloy rims.
Drives straight and handles great.

PROBLEM:
The noise peaks at 60MPH. Can't hear it at 55MPH, can't hear it at 65MPH. Can slightly hear it at 43-48MPH, albeit much quieter. It is a low-frequency hum that pulsates a little faster than once a second, but not quite twice a second. Sounds like: wah wah wah wah.... where "wah" rhymes with "law". Try saying it out loud at that speed. I assure you that's exactly what it sounds like. I felt like a complete idiot telling that to mechanics the past few days. The minute I'd give them the above information (including the vehicle description above) and they'd tell me it's a tire wear problem I knew they'd be of no help. One guy was nice enough to put it on a lift and try wobbling the wheels checking for wheel bearing issues, but he found none. I tried applying the brakes lightly at the above speeds and it didn't change the sound. Also, wheel bearings, as I understand it, gradually increase their noise with speed so I've dropped that route. I've also put the car in neutral on the highway at 60MPH with the noise present and revved the motor. No change in noise. The CV boots are not greasy or torn. I'm a RWD guy so I really don't know much about FWD. Also, in the past week or so, the steering wheel has began to shake just a little at all speeds. Nothing outrageous, it's just noticeable.

In order to contribute something, here's an informative chart for diagnosing noise. Though it is obvously "geared" towards RWD vehicles, there are general issues included:

http://performanceunlimited.com/docu...tionguide.html

The problem I'm experiencing falls in the u-joint categories. I've actually had u-joint problems in the past on my truck. But this is not a RWD pickup we're talking about here and it's nothing like any u-joint problem I've ever had.

There's also some other basic auto information at that site. Just delete the vibrationguide.html from the above link.

Thanks for reading and thanks in advance for any advice.

-Johnny-

azkicker0027 · Oct 8th, 2005, 08:01 AM

well, to eliminate the easiest part, try rotating the wheels front to back.
if you can have the car on the lift running at about 60, you can see if it's from the trans as well. lift precautions though.

john-ee · Oct 8th, 2005, 01:58 PM

Thanks for the response. Tire wear has been ruled out as the tires are new and balanced and I have rotated them. I don't want to try running it in gear on the lift. It needs brakes, so I'm doing that this afternoon. I'll post if that helped at all.

-Johnny-

HATEnFATE · Oct 8th, 2005, 02:52 PM

It Sounds like a tire wear issue. What your hearing sounds like a noise ocsilation caused by the tires being out of balance or being worn unevenly. maybe the tires are getting cupped. It's not uncommon for SP40's (I sell them all day) to have issues.

Another thing it could be is an excessive road force on one tire. Take your car to a dealer that has a road forcing tire machine. You'd be suprised.

have your Tuned a guitar? when getting it in tune the best way to get it right is to listen to the vibration in the note as you adjust it. There will be no vibration in the note between the two strings if they are in perfect harmony. It it's off by just the SLIGHTEST amount...it will sounds like it's vibrating. Tires do that too.

HATEnFATE · Oct 8th, 2005, 02:53 PM

You could have struts going out which would cause the tires to wear unevenly.

john-ee · Oct 9th, 2005, 12:56 AM

Thanks HATEnFATE,

R/R brakes, drums and rotors and the sound is still there. The brakes were shot anyway and I didn't want to pay $600+ to do it.

I completely understand the guitar scenario that you are talking about. As an EE I deal with frequency-related issues all of the time. They're electrical signals, but the concepts remain the same. It's called warble. I guess the tires are the most plausible answer, although I could really feel nothing wrong with them while the car was up. The struts have never been replaced and the tires were not all replaced at the same time (2 front, then two more 2 months later), so even 5k miles could have a significant impact on their evenness.

-Johnny-