yep i have this as well! sounds like a diesel at the front, sounds awesome from the back with exhaust!! MG told me it was because i was low on oil. i did'nt beleive them as i do a service on the car every month.
Do some basic checks like the oil level, and see how dirty it is. As it gets cold now if the oil is low or is a bit thick, (6,000 miles should be the maximum between oil changes in my eyes) then until it is warm it will struggle to get up to the top of the engine.
Your tappets are filled with oil normally, so if oil is not reaching them very easily or quicker they will rattle quite badly. Also if there is little oil getting to the top of the engine you will get wear as mentioned above. If everything looks OK, get it to a dealer ASAP.