Hesitation Problem

I bought my son a 2001 9C1 Police Impala about a year ago. It has the high performence 3800 engine and all the hd police options. The car had 40,000 when we bought and was almost like new. I replaced tires, brakes, Serpentine and that's about it. It now has about 69,000 miles and he says when he steps on the gas it doesn't want to go. He says if he steps on it really lightly it does accelerate but if he stomps on it as you would to pass someone on the highway it doesn't want to go. He also told me it sounded really loud the other day. He brought it to our local service guy and he looked at it and thought everything was fine. Any ideas what could be casuing this? I'm thinking maybe tuneup items, plugs, wires, timing? He thinks maybe the transmission.
thanks
 
 

I had trouble accelerating around 4,000 RPM .It ran fine if I took it easy,but bogged down if I jumped on it.Turned out to be a clogged catalytic converter.There is a documented problem with the 01 & 02 Impala's. It is covered up to 10 years 120,000 miles.

'02 Impala LS

was gonna say the same thing, especially if it is getting louder

I had the same problem before and it was because my gmc catalytic converter was clogged. so better checked yours..  it would also good if you'll have your car tuned-up so to know if there are other parts that needs a replacement. good luck!