Boxerworks.com

1978 R100
Just running it a little today to seat in new rear pads and when I shut it off I heard a hissing noise from the left cylinder only???
I removed the spark plug and the noise stayed there. It did not go away until I removed the valve cover. ???
its strange it was only the left side???
I am stumped, any ideas???

I'll bet you have an opening/air leak on your valve cover gasket. probably near the top, if it's not leaking oil.

Was the bike wet... or has it been wet shortly before?

There is an open cavity formed between the head and the rocker cover. In wet weather or if the machine is washed, water can get in. When the head heats up, it will boil out causing a hissing noise. It will last for quite a long time until all the water had evaporated,
Could this be your mystery noise?

Rob