Help! Strange sound coming from the engine

[bluestreak_96gt]

My low oil engine light came on, (it was ready to be changed anyways) But like a dumb as I got on it pretty hard (racing) Got off the expressway and notice an odd knocking sound. I'm thinking rod knock, (hopefully not). But the thing is it only knocks at about 3000rpms to about 4200rpms under higher rpms. Next day when its daylight I look under to hood, and I'm missing my idler pulley wheel. Son of gun backed off, so now I waiting for ford to deliver a new bolt and pulley. Could the noise that I am possibly hearing be the belt slapping around due to the wheel and access slack in the timing belt? I also changed the oil asap too but still hear the sound. Any suggestions? Thanks guy.