99' GC Laredo - Strange new noise

[cbirdsong22]

Hi,

My JGC (96,000 miles) has just recently developed a weird noise.

The characteristics are:

When accelerating, I get the normal sound of acceleration, and then on top of that there is another low humming noise. It sounds kind of like a car without a muffler would sound, but not quite as loud.

Any ideas?

Thanks in advance!