Does the noise change with road speed, engine speed, or does it stay constant?

You say it goes away when you let off the gas, so that sounds drivetrain related to me. My first guess would be the rear differential, but like someone else here said, it's really hard to diagnose sounds over the internet.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles