You may laugh at this, its just a sugestion. Could it be a cracked/holed exhaust manifold? Leaking exhaust manifold gasket, this can sound very bad, if its got a bad crack. Had it on my Calibra, thought the engine had piston slap. Take your manifold heatshield off and have a look, it would possibly sound quieter when warmed up and would have got worse over time. As for the cambelt, Ive had this on a Corsa fitted by my local Vauxhall garage, only noticed when I had to replace the head gasket meself due to lack of funds, then realised what they had done. Every cambelt after that, even the 147 1.6 ts Ive just bought the misses Ive done meself. The mechanics are told just to hold the tensioner back, slide the old belt off and the new one on. if the tension is out, its out(I dont do this meself!). If youve had a ts belt off youll realise there is no way you could do this, hence the belt being stupidly loose. The cam sprockets on the 1.6 ts I did needed the sprockets adjusting to be properly aligned with the belt and the cams timing with cam locks. Youve been unlucky with yours by the looks of it. Might be worth doing a compression check? That way couldnt you tell if all the valves are closing fully on to the seats? That could give a bad rattle, if theyve colided with the pistons because of the belt issue? Might be worth doing just to be sure? What do others think? Or am I going the wrong way with this?