Here are two completely different vehicles with a very similar problem.
Q: I have a 2010 Chevy Cobalt with just under 50,000 miles on it. It has a humming noise coming from the front when I get to 15-20 mph, noise that gets louder as I continue to accelerate but does not change when it shifts gears or if I turn left or right. I had my tires rebalanced and rotated with no improvement.
Q: We own an '06 AWD V-6 Cadillac SRX that has a YUN, YUN, YUN sound I can feel in the driver's seat. In preparation for a road trip, I told my 30-year mechanic we needed to get this fixed. He discovered a broken pad on the left front and replaced both sides. Still have the YUN YUN. Next he raised the vehicle, got both front wheels turning and determined the left front bearing was not bad. Replaced right and left side bearings but still have the YUN YUN. Next, replaced both front rotors, still YUN YUN YUN. Throughout these steps I have been raising the issue of CV joints but he said the undercarriage was clean and would have had extraneous lubricant if the boots were damaged. The tires are high-end and guaranteed and only have 20K on them. I had the fronts swapped side to side and balanced. Still YUN YUN. Next, replaced the rear rotors, still YUN YUN YUN.
He now feels that one of the pads may be defective and could be replaced through warranty. He asked me to take the car up to speed a few times and apply hard braking. Still YUN YUN YUN. It may be my imagination but it seems that the YUN YUN does not happen in the early part of a drive, only after a few miles.
As you can guess, it's driving me crazy and I hope you have some wisdom that will help.
A: So the Yun Yun is no fun, no pun intended! One of the problems with being a somewhat anal and caring car owner — and this is from personal experience — is becoming overly concerned and frustrated over an issue that is as much annoying as problematic. When this occurs, one tends to develop tunnel vision and focus on just one potential cause.
Looking at both cars, if the harmonic or rhythmical noise/sound is related to wheel speed, the likeliest suspects are tires, wheels, wheel hub/bearings, drive axles and brakes. Even something like torque converter clutch slippage could be a possibility.
The tires are still a strong possibility so I'd suggest checking all the wheels and tires for run-out and out-of-round, then have them road force balanced. Make sure the shop checks the wheels themselves for these issues. Although the Cadillac AWD vehicle comes equipped with different size tires front and rear, check to make sure all four have the same rolling diameter. Mismatched tires may well be the problem.
One potential cause that may not have been checked is a dragging brake pad caused by a sticking caliper slider or piston. The clue that may point to this is the noise developing after driving a few miles. You might be able to pinpoint this by driving 10-15 miles in urban stoplight conditions, then quickly pull over safely and sense the temperature of the each wheel as close to the hub as possible. An inexpensive infrared temperature gun is perfect but a quick touch with the back of your hand will identify any significant differences in temperature, which could indicate brake drag.
The front drive shafts/CV joints certainly could be the cause due to wear (they don't have to leak grease because of a failed rubber boot to develop enough wear to create an issue). With the car on a hoist, a strong arm can push/pull in/out and up/down to identify excess play in the shaft or joints.