Hello ive had my car for 6 months, i recently had the timing chain and exhaust valves changed/repaired and since ive had it back when accelerating in low revs e.g. accelerating in 2nd gear at 5mph there is this grinding sound that comes from the engine, it even happens in 1st gear if i dont take off fast enough. My mechanic took it back as an engine light was also on but he said it was my cat which was rattling/in need of replacing. It is still happening after he returned it. Can anyone tell me whats going on?