I recently noticed my car is making a barely-perceptible, low-pitched, buzzing or droning sound at highway speeds. It kind-of sounds like some cars' exhaust, but it's quiet enough that I seem to be the only one in my family who is hearing it. (It's about on the same volume level as natural "road noise".)
The sound only begins to be noticeable around 45 MPH. Once the car is going fast enough that I can hear it, the sound does not go away until the car has slowed to below the same threshold.
The sound does not follow, nor does it seem at all affected by, the engine's speed. (I've listened during normal usage as gears shift automatically, and also forced up/down shifts and shifted to neutral - none of these cause any change to the noise's pitch or volume.) It also is not uniquely affected by braking - so long as the car continues to travel faster than about 45 MPH, it remains present and at its usual pitch and volume regardless of whether or not the brakes are applied.
I've tried to localize the sound to no avail. It feels a little like it's coming from the front of the car, but the other day I could swear it seemed to be coming a little more from the rear. Since it only happens at 45+ MPH, and is fairly quiet to begin with, listening from outside the car is not an option and it gets drowned out by the wind when the windows are down.
The sound only seems to be slightly affected by vehicle speed. It just barely raises or lowers pitch as the car goes faster or slower, but the volume does not seem to change much.
The car in question:
2012 Dodge Avenger SXT
2.4 L inline-4 DOHC engine
6-speed automatic transmission, FWD
Brief recap of sound characteristics:
- Low pitch, low volume. (Barely noticeable against road noise.)
- Occurs while vehicle speed is >= approx. 45 MPH.
- Unaffected by engine speed, gear selection, or braking.
- Pitch has a slight direct relation to vehicle speed.
Given that the sound seems to be somewhat related to vehicle speed, but independent of engine speed, one of my first suspects was the wheels or tires. However, the tires appear to be in good shape and (recent weather fluctuations aside) don't seem to have problems holding air.
What are likely causes of this type of sound that I should be looking at?