i removed the stock white washer in my 2007 S40 and replaced it with a nylon washer that is a little more than half the height of the original.  The bike runs better, but now I have a ping under load.  Higher octane gas reduces the ping, but does not eliminate it.  Anyone else experience this?  My next step is to reverse the mod, but it does runner better, except for the ping.  

The ping sound more of a chirp, in fact I am still grabbing the headlight to see if it rattling.  I stopped the rattle in the speedo with rubber bushings.  

Any thoughts appreciated.

Check the exhaust to see if it is tight at the cylinder.

thanks.  the exhaust is tight, but good thought -- i might check it with a flame.  I'll check the drive pulley as well.  if nothing else, I may take it apart again tonight.

It was my belt!  I took it on a ride with my face shield up and I could hear better.  And, the ping/knock was really more of a chirp from the rear of the bike.  I parked the bike and started poking around it and found that the rear sprocket resonated when poked at about the same pitch as the knock I was hearing.  I used the tension tool and observed that there was no compression measured by the tool.  So, I adjusted it -- using both the tool and the suggested 90 degree method advocated by so many here -- and the knock/chirp is gone!

Weird timing of the knock/chirp, and I was really fooled.  Never messed with a belt drive before -- great lessen.  

By the way, the #4 washer I installed, which is little more than half the height of the original, is still in the bike, and without the now corrected belt chirp, the bike performs better and is great to ride.