I noticed a low humming noise lately on my wife’s 2007 Outback which started at around 40 MPH or so. I thought it might be something with the added roof rack attachment or tires which are Michellen Rain Force tires installed about a year ago. It sounds like an aerodynamic issue to me. Nothing has changed on the roof racks and the tires show no sings at all of unusual wear. I looked in front of the vehicle and underneath to see if I could see something and noticed the cowling immediately under the exhaust manifold and where I removed the small door for frequent oil changes is bowed down in the rear creating a scoop which I think is the humming noise. I am trying to find the darn door in the garage so I can re-install it, but it got me thinking, is the whole cowling even necessary to have under there? I know it can help with rocks and things from hitting the underside of the engine, but is it really more for simply aerodynamics? It is a pain sometimes working underneath because it blocks access to everything. I was thinking of simply removing the entire contraption.
It’s necessary, keeps water off the engine. Better to fix it.
The bean-counters employed by car companies frown on putting extraneous equipment on cars.
If it didn’t have a legitimate purpose, they wouldn’t have spent the money for that cowling.
In addition to adding a tiny bit of aerodynamic efficiency, that plastic cowling also helps keep water from damaging electrical/electronic components under the hood–as texases stated.
It keep water out of the engine. There has been a couple posts where the cowling was missing and then the alternator would get wet. This caused the car to cutout. Both were Hondas if I remember right.
Wheel bearings start making noise too at that speed.
Andrew made a good point. And when my alternator was failing last year it started with a humming noise.
Have you popped the hood and run the engine to listen for a point of origin? It might not be an aerodynamic issue.