I don't know about the K1 models but the later K3-K6 models have a PAIR emissions system. It injects air into the exhaust manifold to burn up any unburned hydrocarbons in the exhaust. That can cause a pop when you close the throttle.
--- Craig AKA Cliffhanger----
"Big Red" 2007 Red Executive - 122,038 miles
"Little Blue" 2006 Blue 400S - 43,942 miles
"STING" 05 B650 gone but not forgotten, RIP