i hope someone can help shed some light on my cars problem

it is a 1995 940 t diesel estate. 2.4? audi engine.

over the past year there has been an intermittant rattle off the engine that sounds deep and rattly , it is not always there but is especially noticable at low revs.
recently it has been burning much more fuel and emitting more smoke on acceleration but normal on criusing.

this rattle is present more often than not and doesnt appear to be related to any particular pattern or rev rate.

is this a common problem which occurs?
i brought it ot a friends dads garage and he said he'd have to strip down the engine and it then it may be a big job parts wise.
another option is to buy an equivalent car/ saloon/ estate with an ok engine and replace the engine . i also need a new slave cylinder which could cost too

any tips advice would be welcome