Engine Knock

I have an automatic 2000 Maxima, recently I put in a cold air intake and underdrive pulley. When I put the car into gear and use the accelerator without letting it warm up for a minute, once in a while I notice an initial loss in power accompanied by an occasional engine knock, but only at first, then it runs fine, idles fine when not in gear as well. Have not modified the ECU at all. PLEASE HELP!