2001 Chevy Lumina 2001 Chev. Impala loud noise/vibration

Noises problem
2001 Chevy Lumina 100000 miles

I have a 2001 Chevrolet Impala with over 100,000 miles. My car drives and handles well but has become very loud. Most of the sound seems to be coming from the engine. There is also slight vibration. The noise is NOT present when idling, and only fully evident when the car goes over 15-20mph. The noise did not start instantly but became louder and louder over a year or so.
I have had the car serviced and it receives regular oil changes, etc. Most recently I had the exhaust replaced (a mechanic suggested it may be the cause of the noise). Unfortunately the noise is still there. I have several friends suggest possible causes, including a bad CV valve, or low transmission fluid, or simply that the engine mounting needs tightening.
Any idea of what it could be?