1997 Volkswagen Golf Engine Rattles

I've had a few issues with my car recently, it's had a rattle for a while, I've replaced my back box to a twin pipe. But have wondered whether it's my cat... I had it looked at underneath but mechanic said it looked fine... However I have recently had startup issues and had a new carboretta gasket fitted while having a full service at its MOT; but this has caused more startup issues (possibly due to cold conditions / wrong oil ) and the rattling seems more pronounced and struggles to start when it rains... Especially when the engine is cold...