2003 Echo - tuning fork sound from car at speed. what is it???

[starridersmn]

Bought a 2003 Echo used from some folks in Feb. It has 75000 miles and they had all maintenance done at a dealer.
FWD, no power steering, manual windows, auto, has AC.

Car is in great shape. Brakes were new in Januaryish.

Today when leaving work, I got up to speed on the highway and I heard this harmonic 'tuning fork' sound coming from what seemed to be the underside of the car - kind of in the middle to rear.

I thought it was the AC running (90's here today) but even when off it still did it. It occurred more 'at speed' but did it at low speeds too.
Need to play w/ it more for a more accurate diagnosis, but, anyone know wtf this is?

At some points it was really loud.

Any help appreciated. Car has had no issues and did not make the sound on the way to work. I pondered maybe due to how hot it was today it tied in (?) but then that seemed unlikely.

Thanks.

John in MN

[2005RS]

could it be a front bearing humming? taking a bend or weaving a bit when there's no traffic will cause a bearing hum to change or stop. you're at about the right mileage for a worn out bearing