2003 Nissan X-Trail Engine Misfiring at Idle

After recently servicing the car & changing spark plugs, Oil, Oil filter & air filter. The car car began to misfire at idle. It made a popping sound from the exhaust every 3-5 seconds or so. What could be the problem because it was not misfiring before the service. All the plugs are gapped to the right spacing. Recently it has been running a little noisier but still misfiring.