Hum from under the hood - what could it be?

I have a '99 Dodge Stratus V6 with 125,000 miles. When I turn on the car, I can hear a hum when idling. When I start driving, I cannot hear the hum anymore (but maybe it’s drowned out by other noises the car makes, or just the engine). It gets higher pitched as it slows down to idle, and a bit lower pitched if I step on the gas lightly. It has been slowly getting louder over the last few months. What could this be due to?

Have you checked your power steering fluid?
If you are low on fluid, or if the PS pump is on its way out, this type of noise can be the result.

Yes tis could be your PS pump whining. OR hmm cold be a lot of things really, very hard to diagnose this way but I will try. If you dont ave the radio on and you are at idle you might be hearing your fuel pump. Try this…get in your car tur the key to thr run position but DO NOT start the car. Do you hear it then? If so thats your fuel pump…some pumps make more noise when they begin to wear out. If you dont hear it start the car and see what you get.

On another note, you also seem to be describing what we used to call Radio hum…I used to install custom sound systems with amplifiers in them. If you routed the wirs wrong you would get interference from the ignition system being broadcast over the speakers of your radio. It will get deeper in sound the slower the engine spins…the faster the higher pitched the hum. Did you have any radio work done recently? Make sure your radio is turned off and not just down. Let me know what you find, I can guide you further with more info.

Could also be your alternator. Try taking something solid like a tire iron or pry bar, (or mechanics stethescope if you have one) hold one end to your ear, the other end carefully to the alt. or ps stump. while the car is running. Should be able to tell which is making the noise.

Try removing the air filter and see if noise is gone.