noise when rpm drops from around 1900 to 1400

[yeehoo]

93 k1500 4.3 tbi, I'm getting a playing card in the spokes kind of sound when my engine rpm drops through the 1900-1400 range. It sounds valve train related to me. Has anyone had this problem? Any advice/suggestions on how to pinpoint the exact cause and how to fix? Also, I get a valve or lifter tap from #4 cylinder when the engine is cold but once it warms up the tap goes away. Thanks in advance for any replies.

 

[Schurkey]

Exhaust backfire on deceleration? Does this engine have an AIR system?

 

[yeehoo]

Not sure what AIR system means. It has a functioning A/c system noise is regardless of whether ac is on or off. Its a reman with 2k miles on it.

 

[Schurkey]

AIR = Air Injection Reaction = atmospheric air pumped into exhaust stream under certain conditions, promotes continued burning of Carbon Monoxide (CO) and Hydrocarbons (HC) in the exhaust. Not all trucks had AIR. Most used a mechanical, belt-driven pump. Some had an electric pump.