2002 Camry knock sound and burnt smell at start
 

I just bought a used 2002 Camry that has about 65000 miles and single owner. When I start the car in mornings, there is a mild knocking sound below the hood and some burnt smell in the exhaust, for about 3-5 seconds, and then everything is normal throughout the day. This happens only the first start of the day in the mornings. I took it to Midas, NTB, etc, and they all say it is just some fuel injection system having carbon that burns away in the morning during start. They think the slight knock sound exists in almost all cars that are this old, and there is nothing to worry about. I have been noticing this for last 3 weeks, and there is zero oil leak, the engine and transmission oil levels are full. Is there anything I have to worry about?
Thanks in advance.