My car has a rattle coming from it, it's 2010 and has 114,000 miles on her, a guy said it's the timing chain, but a woman hit my car last year slightly and I've heard it since then, would it be advisable to change my timing chain? Roughly how much would this service cost?
Based on what you've said, that is a hard question to answer – but my best guess is no. The coincidence of the collision and the rattle suggests that the noise has another source, it is unlikely that the collision would have caused a problem with the timing chain.
Your first step should be to do a bit of research. For peace of mind, I'd recommend that you check the recommended service intervals on your car and see if, and when BMW recommends replacing the timing chain. Chains can wear out, but they are more durable than timing belts for the most part. If you had a belt I'd be thinking that a replacement would be in order, but for a chain I'm not so sure. If you're not due for a chain and you've been reasonably good about keeping up with oil changes, I'd put that one out of mind.
After that start paying attention to the rattle, start by trying to identify where it is coming from and if there is a pattern to when you hear it. Also listen for changes, if it is an engine sound it will change in frequency with the engine speed, some sounds may also be present when you start the engine, but then go away after the engine has run for a while.
To further put your mind to ease about the timing chain, open the hood while the engine is running and listen to the noises. The timing chain will be at one end of your engine (probably the front it if is a conventional engine in a longitudinal installation (front to back), although I think at least some BMW engines have it at the back. If the noise is coming from the engine and seems to be related to the timing chain, then that would be worth exploring more.
Since you think the noise is related to the collision (or since I think it may be), pay particular attention to the area where the car was hit. It could be something like a loose body panel. Regardless, just listen and try to see if you can find a pattern and an area of the car to point to. Then use that information to refine your question – or to explain to a mechanic how s/he can help you.