2003 Monte Carlo SS w/ 3800 V6 96,000 miles

My in laws gave me their 2003 Monte Carlo w/ a 3800 in it in Dec. 06. For the last few weeks it seems like it takes forever to get to 40 mph. It runs fine between 40-60 mph but if you try to speed up quickly it makes a horrible sound. The harder you press the gas pedal the louder the noise and as soon as you let off the noise stops. The closest I can come to describing the sound is maybe a grinding/knocking noise. That doesn’t really do it justice but no one can think of another way to describe it. I've also noticed that the oil pressure gage moves a lot while accelerating and slowing. This may be normal; I've just never had a car that did as much as this one.

I had a family friend who dabbles in car repair take it for a ride. He has no idea what it is but said it sounds like it is coming from the bottom of the motor. He doesn’t know were to even begin checking. He also said it should have more power due to the size of the motor. I would appreciate any useful tips on where to start. Thanks