"Rumbling" sound with Elantra

I have a (2004, I think) Hyundai Elantra, with ~11000 miles on it. I've recently (last couple of weeks) noticed that I am hearing a
"rumbling" sound that seems to be coming from the front of the car when it is moving (we think it's from the front, because my daughter said that she didn't hear the sound when sitting in the back seat). I mainly can hear this sound when I take my foot off the gas pedal (i.e., not accelerating), and the sound continues even when the transmission is moved from "D" to neutral, and continues "almost" until the car comes to a stop.
I don't know if it's just a coincidence, or if I'm just more sensitive to sounds after the last oil change (about 2 weeks ago), but I don't recall hearing this sound until after I got the car back.
Is anyone aware of something like this with the Elantras? What could it be?
Thanks in advance, Jim
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