low-frequency pulsating hum noise
 

for the last few months i've been hearing a low-frequency pulsing hum. i say pulsing because it goes *hummmm*...silence...*hummmm*...silence...etc at about 2 second intervals. this hum fills up the entire cabin so its hard to pinpoint where it originates.

this is most audible at 2500-3500 rpm cruising in 5th gear going anywhere from 55MPH to 70MPH. at the higher rpm i'm not able to tell if it occurs because the engine/road noise blocks any low frequency noise.

the car has < 75,000 miles, recently put new front and rear brake pads although i am not feeling any notible NVH from the new brakes themselves. no exhaust work has ever been done, no engine mods.

any ideas are appreciated. thanks!
-meGrimlock

First rotate the tires. The fronts and rears wear differently and tire noise from the rear is louder in the cabin that noise from the front wheels. If the noise changes or goes away, you know its the tires. If not then it might be a wheel bearing but not too likely unless you hit a pot hole or something really hard at one time.

the tires were rotated about 4000 miles ago (going to rotate when in another 1000). i'm not exactly sure when the noise started, but it might have been around that time.

i had a set of bfgoodrich g-force sport tires that were noisy as hell and made a much faster humming noise on the highway. so it could be the tires. thanks for the insight.
-meGrimlock

did you ever find the source of this noise? I have the same exact problem and I'm a mechanic... just cannot put the finger on it!

we have done swap with another Mazda 3:
- all wheel bearings
- both axle shaft and jack shaft
- transaxle
- brakes and discs

still have the big noise and vibration....

any help would be great!

balance the tires... they can get in and out of sync since the tires aren't exactly the same diameter. If this is the problem you should be able to hear a difference when going around a corner, since they will sync up differently during that time.

Drive in 4th gear to see if you get it at the same RPM, or the same vehicle speed - that will narrow down the source.

Hi, did anyone get to the bottom of this problem discribed by meGrimlock. I have exactly the same noise at the same sppeds, its driving me mad and spoiling an other wise nice car over in the UK?

Thank you for any responces

Prop Shaft may be the issue
 

I've seen this issue pop up on several other forums and it looks like several folks have had the issue resolved by replacing the prop shaft. I'm sitting at the dealer now with my Mazda 3 Turbo Hatch with 800 miles on it, waiting for them to check it out. The service tech said that he didn't hear anything out of the ordinary. Now I guess I've got to get them to admit that there is an issue then they can begin to figure out what is causing it.