I own maruti 800 november 2005 model 4speed gearbox a c

I own maruti 800 november 2005 model 4speed gearbox a/c version, which has covered 1,10,000 km without any engine problem at interval of every 50,000km i clean fuel injectors from lucas service center. At every 5000km i change: engine oil, gearbox oil, air filter, oil filter, fuel filter, spark plugs. Now the problem has arise at idle and speed below 20kmph engine make noise like diesel engine car, is it a engine problem or anything thing else i have recently changed engine foundation but is still there..