What could be causing pinging/detonation?

First off have you adjusted the timming at all yet? Second what grade of gasoline are you running? On AVG our cars will take 13-14* of timming w/ out pinging, of corse thats only if you use 93 Octane. Even if your timming is set at factory settings it could still ping w/ 87 octane gas. Getting a timming light or find someone that has one and check your timing and see where its at.

If you think you are running lean stay off the gas until you can get a wideband o2 sensor hooked up, thats my advice. I didn't stay off the gas and now my motor is on an engine stand.

Timing, bad gas and heat range plug to start off. Try checking those and see if it stops.

I had an issue last year with pinging, when I was stock. I'm 99% sure the problem was the IAT sensor, located in your cold air intake, or factory air intake. Mine wasnt straight in. My sensor was blocked by its surrounding "shield". The air wasnt hitting the sensor properly and was making the car ping above 11* on 94octane. Just unscrew it, it takes a minute. You'll see what I mean.

I had locked timing @ 20 degrees when I was running at 6 psi of boost and my car pulled harder than ever for me and my pinging exactly as yours stopped. I am now @ 8 degrees locked timing and I have not heard any detination yet.