motor "burps" when cruising

[funkingonutz]

my 3.0L (95 120,000) likes to burp randomly as im going down the highway, its very hard to notice it, only i notice it and none of my passengers, it happens only sometimes, and its only when im criusing (not accelerating) either down the highway or a long road, at a set speed.
what the hell could this be??

 

[gm3fan]

If you find out let me know. Mine only does it with the a/c on and cruising.