l bought a second hand Saab and now it has a rattle in the engine and when you start it up it is quite loud.

there might be a clot inside the engine or most commonly oil gettin absorbed very quickly.

It sounds like the timing chain tensioner. If it is one of the classic (1979-1993) 900 is then that is almost certainly the cause. It does not cost much to get done, but you should get it done sooner rather than later as eventually it will cause the timing chain to snap.

Give yourself a pat on the back too, I've had four of the classic 900 is they are brilliant! Loads of personality and will run FOREVER

What kind of rattle is it? Does it sound like it is coming from inside the engine is camshaft cover or somewhere else? It could be a sloppy timing chain rattling around in there which is not too healthy, or it could be as simple as a loose heat shield rattle against something.