Not sure but from what you describe it would be worth getting it looked at fairly quickly. Could be a fuel problem or maybe electronics. Could you just confirm the chugging sound is in low gear, at low speed, or low revs? Forgive me if I am being patronising but when you take your car to the garage, be sure to tell them not to do anything with out telling you what they are going to do, and how much it will cost. If not careful you can end up with a large bill for them to say they tested it and didn't find anything! Issue specific instructions not to do any work without your specific authorisation.