According to Official Guinness Records,
Kunal Jain (Canada) generated a sound level reading of 114.2 dBA by clicking his tongue at Richmond Hill, Ontario, Canada on 6 August 2003.
As a comparison, a lawnmower is on average 90dBA and a car horn is 110 dBA.
For a complete list of 2003 records, please visit 2003 Guinness Records in Canada.