99 Silverado Popping While Accelerating

[meronekp]

i have a 99 silverado 5.3 z71. All of a sudden with no provocation my truck decided to make a very annoying and embarassing noise, and has not quit since. the sound it is making is difficult to explain, the best i can say is it is a mechanical sounding pop that at times sounds like when you are breaking loose a very rusty bolt and you get the chirpy creek. the noise will be there for a few seconds at startup when cold then it goes quiet and no amount of revving will cause it to return, until you put it in gear and increase engine rpm at all. in either direction touching the gas pedal causes this constant and loud popping sound. the sound does increase along with engine rpm, however not equally. i mean if for example when it starts say it is popping 25 times in 10 seconds but if you double the rpms it will go up but maybe to 30 in 10 sec. initially i thought i had gotten a branch caught where it could hit the fan blades, but upon inspection and as close of listening as is possible, it sounds to be coming from the passenger side of the engine. any ideas would help significantly. other info i do have 3 recurring codes of the po449 and p0420 for the nice canister valve problem and on occasion i get a p0405. help!!!