Several days ago while driving my car, a revving sound developed. I thought perhaps it was the engine revving, but I noticed that the RPM gauge was normal. Today, the revving/humming noise begins after I’ve been driving about 20 minutes and the sound lasts about 6-8 seconds. It will repeat this by continuing on/off for about 5-6 times and then the sound stops. The car drives well and no problems to report with the starting, accelerating, braking, etc. There are no lights on the dashboard. I took it to a mechanic and he couldn’t find the problem. In your experience, what is causing this revving and humming sound?
It sounds like your cooling fan kicking on and off. It’s even louder and more pronounced when the AC is on. The strain on the engine also will cause the RPMs to jump up a little bit during these cycles. If you would like to have this checked, a qualified professional from YourMechanic can come to your car’s location to diagnose the noise and recommend a correction.