i'm getting some chatter from engine compartment when I move into passing gear.

What is your question?

At speed, (2007 Lucerne) when I go into passing gear via foot petal their is chatter from the engine compartment. Only happens then so can't check it out. What do you think? Water pump....fan.....internal engine......transmission? 88k good power well kept. Transmission fluid not changed......Water pump and coolant changed two year ago. help.