Please help me figure out this noise..

[HotRod73]

Well I am lookin at buyin ANOTHER truck... this has a weird noise... when you first start it up you hear this weird noise KINDA like a rod or exhaust hitting something... but then goes away and i let it warm up and everything and i never heard the sound again... what could be the cause of this? the rod? the bearings? the exhaust hitting something? (i couldnt see exhaust hitting anything but it was dark) only does for a few seconds when u first start it is when i noticed it...