92 LS 400 Pinging

I have a 92 LS 400 with 172,000 miles. Recently it has started pinging. It seems to ping after it gets warm and only between 1,000 and 2,000 RPM. Beyond that the pinging goes away. I have checked the valve clearance and they seem to be under spec. The timming is correct as well. Any ideas?

Could it be regular gas? I hope not as that
is what I plan to use in my recently aquired
'90 LS400
Probably an anti-knock sensor not telling
the computer to back off timing advance.