Thump on start

[westex39]

2007 Chevy Suburban, 5.3L, 2wd. Occasionily on cold start there is a fairly loud thump just as the engine starts. It's not engine internal noise. This noise only happens once. Successive starts are no problem. I thought it might be torque converter drain down. I have never had this problem before. Do any of the forum members have any ideas?

 

[JustinH]

Piston slap?

 

[Brett Miller]

A/C compressor?

 

[Triple_Se7en]

Fan clutch?

 

[Curious Kid]

Engine mounts being cold and thus more stiff transferring the start torque of the engine catching?