> "Pinging" or pre-ignition knock or "spark knock" is real and happens during acceleration due to the fuel igniting before the spark plug fires. It's usually brought on by heavy engine load (acceleration) and a lean fuel condition, not enough octane in the fuel, or possibly a spot of red hot carbon in the combustion chamber. But "spark knock" is always on acceleration, never deceleration.

> There can be a type of "back firing" on deceleration. Large amounts of unburned fuel enter the hot exhaust on deceleration. If there is any air leaking into the exhaust system (such as at the cross-over tube), then the introduction of oxygen will self- ignite the fuel inside the pipe. Since it is muffled inside the pipe, it sounds like a subdued pop, pop, pop. You can look for tiny black areas where jets of exhaust have been leaking at the pipe joints during normal running. If exhaust can get out during normal running, then you have to assume that air can get in the same way on deceleration. Late 70's bikes did not have exhaust clamps on the cross-over pipe, so this is something to consider.

To prevent this situation, I always fit the cross-over pipe with silver silicone sealer when I'm assembling the exhaust system on an Airhead. Even normal silicone sealer will last over a year in this environment, and the silver color helps it blend into its surroundings so it's not so obtrusive. This is something to look for, especially if you've had the exhaust system off the bike in the last couple of weeks before this event. And a lot more cost-effective than tearing into an engine for "investigation" !!

Hope this helps.

Thanks for the reply! I had not considered a leaky exhaust.  I think mine is ok.  To add, I checked the right side valves and they were in spec.  However, I took it out this morning and after about 15 minutes the sound came back (coming to a stoplight) a 2 second winding and releasing sound.  Then again and suddenly my valve is clattering big time.   Pulled the cover and the intake valve was way loose, like at an 1/8 inch.  I am able to set it again in spec.  The bike has had valve work 15,000 miles ago.  I think I have valve troubles on that intake valve.   

Yikes!  Be sure you let us know what you find.

I presumed the valve work included new seats?  I’m thinking it’s an issue with the valve seat, perhaps. I wouldn’t touch the start button in the meantime. 

Attached picture of trick I used to pull a head for repair without removing the cylinder from the crankcase, if it helps you at all.

There have been issues reported with the bearings in the rocker arms going bad. If this happened, then the valve adjustment would be equally loose. So the cyl head may not need to be removed if this is the issue and internal damage would be very limited or none. This failure damages the rocker shaft, but all that's needed is: new bearings, new shaft, and a check of oil delivery to the rocker arms.

What I'm saying is inspect very carefully during disassembly, and don't get in a hurry.