W202 metallic chatter on acceleration

[Hocky]

I have this used car for the past 5 months, has 189,000kms on the clock and it has absolutely no power and makes this metallic chatter on acceleration , noisier when cold and the exhaust sounds too loud too. Its a 1994 C180 model 4 cyl 1779cc engine. Books don't show any serious engine or trans work done to it. Wonder if this is "normal" for such a low power engine pulling such a heavy weight or its time for an overhaul??? Running on 3 cyls or needs new exhaust system or the auto trans has had its day or what? I am appealing to the techs if you can tell me whats wrong. As far as I am aware of most cars, they usually purr away smoothly even when cold, but this one makes this irritating metallic chatter like "clack, clack, clack,clack for as long as I have my foot on the accelerator while getting away from stationary position like red lights or parked or uphill, otherwise its normal. Do I need to send for computer diagnosis or just rely on the tech's ears??? Please help soon, thanks.