My first impression is that it's a tire. I had a volvo that made an odd rhythmic noise that could only be heard at times, particularly low speeds. Couldn't isolate where it came from. Sometimes I'd feel it when braking or sometimes with my foot on the gas. Most times I could only hear it. Could never figure it out. Then one day, going around a turn at low speed, POW, blowout on the right rear tire.
As soon as it happened I realized that's what the noise was. I was lucky, because generally FLEW around in that car.