I have a 2004 Silverado 4x4 with the 5.3 V8 and automatic transmisson. I noticed what looked like a plug on the bottom of the transmission fluid pan. This plug has a tapered hex head and my English and Metric sockets just slip off. Is this really a drain plug for the tranny? And does it require a special tool? Thanks in advance for your answers.

 

Do you have a pic?
It should be a drain plug. No other reason to be there, unless it's a line opening for something. But you should have 2 then.