Mysterious Clunk

[85Silverado]

ok....just took emissions and passed with my '78 3/4 2wd...but left the place with a problem....soon as i put it in gear and just after I take off and apply the gas there's a clunk that seems to be coming from the frontend or under my feet...I can feel it on the floor and in the pedals when it does clunk and it's only one clunk. It's not a rapid succession of clunks. It does it occassionally while driving down the road but always when taking off and sometimes when i change into second and press the gas again. It won't do it if the truck is standing still and i rev the engine in neutral so I don't think it's a motor mount. I got home and bounced on the front end of the truck cycling the suspension and nothing, no noise besides some squeaking. What do you guys think...any info would be appreciated.