Fan-like noise

2007 Subaru Legacy sedan, automatic, regular maintainance, 40,000 miles or so, makes a noise that sounds like something caught in a fan, or like a card in the spokes of a kid’s bicycle wheel. It only makes the noise when it’s in drive or reverse, and is stopped, as at a light. As soon as I put the car in neutral or park, or move from a stop, the noise stops. It happens whether the fan is on or off. Sounds like it’s coming from behind the glove box. My regular mechanic has been unable to find it. What could it be?

Normally, I would say that the noise is coming from the HVAC fan, and that either there is something (like a leaf) caught in the blower or that the resistor pack is making noise.

However, since the noise seems to be related to vehicle movement and to putting the car in gear, my suspect would be the viscous coupler that transfers power to the rear wheels. Yes, I know that you said that the noise is coming from behind the glove box, but with unitized construction, noises can be “telegraped” to areas other than where they are actually emanating from.

The viscous coupling is normally trouble-free, except if the car has been run with mis-matched tires or if it has been driven for too many miles on a donut spare tire. Was this car purchased by you as a new car, or did you buy it as a used car?

If this is a 4-cylinder Legacy, rather than a 6-cylinder model, you can test my theory about the viscous coupler by inserting the fuse that converts the car from AWD to FWD. Consult your Owner’s Manual for the details on the location of the fuse. If the noise goes away, then that pretty well confirms that the problem is the viscous coupler.

The good news is that, if your Powertrain Warranty is still in effect, the warranty will cover the repairs–as long as the car was correctly maintained and as long as there is no evidence of it having been run with mis-matched tires.

Thanks- I will try the fuse. The car had about 20,000 miles on it when I bought it. I know that since I’ve had it, I’m a fanatic about maintenance and tire pressure, but who knows what could have been done to it before I had it. I did buy it from a Subaru dealer, and have driven for about 2 years before the noise started. When I bought it the tires were good, and I have since had new ones put on maybe 10,000 miles ago. One had gotten cut by something in the road, and I replaced all 4.
Thanks again!