Automatic ransmission clutch packs will chatter when used beyond their service life. The chattering noise is caused by too much tolerance between the friction plate and the steel plate. Changing the fluid at this point will only make it worse. Another thing, although I'm not sure if this is applicable, is the ansmission drive chain. Like timing chains, these will slip on the sprockets as they age and strech. Many transmissions using a drive chain do not have slack adjusters, so they require a replacement. What engine/transmission does this vehicle have? I may be able to provide more info...