Backfire-Question!

[Sooner1970_K10]

I noticed today that my pickup has a small backfire: like pop, pop, pop whenever I down shift and the rpm's spike or I am accelerating and let off the gas in 2nd or 3rd gear. I drive a 1970 chevy with an originial v8 350 and a 4spd manual transmittion. It doesnt have headers and I dont really know anything about the exhaust system. I have only had the truck a couple of months and only drive it on the weekends or once and awhile in the afternoon when I get home from work. This is the first time I have noticed the backfire, is it normal or do I have a valve seat or exhaust problem? Anything I should check out to eliminate? Thanks again for any advice!