Have you recently tipped it onto its side to check the blade for example. If so you may have an excess of engine oil in the exhaust.
Check the engine oil is topped up to the correct level.
Run up the machine and if the problem clears in a few minutes it was excess oil in the exhaust.
If it does not clear switch off and dip the oil level. if it has significantly dropped there may be an internal oil leak which would require proffessional attention
Phil UK, July 2007