Whirring/growling noise - transmission issue?

Mitsubishi Lancer, 2003, 140k miles, auto transmission. About a week ago, I noticed a new noise coming from the engine compartment. It’s not very loud, only a little louder than regular road/engine noise. It starts up a second or two after I begin to accelerate, and it winds down a second or two behind the rate at which I slow to a stop. It is constant when I am travelling at speed. The sound is something between a whirring and a growling noise, almost exactly the sound you hear when knobby bicycle tires roll quickly over smooth pavement. The transmission fluid was about a quart low, and that got filled up, but the noise hasn’t changed or stopped since then. I haven’t had any problems with shifting out of/into gear. The car does occasionally “slip” out of gear in that when I am travelling slowly (stuck in traffic, usually) and going back and forth between light acceleration, coasting, and braking, once in a while I’ll press the gas and the engine will just rev, but it won’t engage. If I just let off the pedal, it will “reset” and then it moves just fine. It’s done that ever since I bought the car in 2009, and it doesn’t do it all that often, and only at those low traffic jam type speeds. So, I don’t know if it’s related to the noise, which is what I’m more concerned about.
Any ideas?

If your transmission level was low then you have a leak and have to have that checked. Transmission fluid doesn’t get used up or burn off the way oil might, so low fluid means leaking.

There are lots of things that can whirr/growl. Does the noise change at all when you make a turn? I.e. get louder or quieter or change pitch? Does it change at all with speed?

I’m sorry, it sounds as if your transmission is on its last leg. It’s time to think about new car. !40k on a Lancer is a lot. I don’t think it is worth the money it’s going to cost to fix it.

@cigroller - the noise doesn’t change at all when I make a turn. It does change with speed in that it gets louder as I speed up and gets quieter as I slow down. When I am completely stopped, with the engine still on (like at a stoplight), the noise is gone. It starts up again when I start moving, though the sound lags behind the movement by a second or two.

I was more concerned with the dropping out of gear & or slipping. Whirring is the least of your problems. What happens if you try to cross in front of someone and it pops out of gear and they T-bone you.

I don’t know of the severity of the neutraling issue, and more or less dismissed it since you’ve been driving that way since 2009 when you bought the car. You might want to ask about it at a local, reputable transmission shop when you have them check for a leak.

My first guess on your noise is just going to be some uneven tire wear. Since that is basically caused by tire balance or alignment/steering/suspension problems I’m going to suggest that you take it to a locally owned shop that specializes in alignments. Have them test drive the car and check the tires and steering and suspension.

Likely a transmission problem. Another idea, a noise sort of like a knobby tire rolling, that gets louder when going faster, and quieter when going slowly, could also describe a front wheel bearing on the fritz. Most people say it makes a “growling” noise. Ask your mechanic to rule that out before assuming this is a transmission problem.