pinging

Pinging started yesterday. All fluid levels are fine, so I am stumped. What do you all think?

Might be a silly question but.... With a helmet on & ear plugs in how would one tell if the engine was pinging??? Just wondering. Sean built a 1300 venture for me and put the high compression pistons in. I have been running 89 but not sure if I need to run 91. Thanks in advance. Cheers