If you have an automatic transmission the noise could be the rear transmission mount sagging. The transmission case will bump and/or vibrate against the engine carriage a little. Right engine dampener will vibrate when it goes - metal to metal contact that will change pitch. In the left wheel well look through the opening between the transmission and the power rack. The dampener is kind of like a donut with a star shaped rubber isolator attached between the engine block and the engine carriage. If the bolt is no longer in the center the rubber is broken and there is a big noise maker that will change with speed and how the vehicle is riding.
