GEN 2 Knocking under load

Hi everyone,

I own a 2006 Prius with 243,000 miles on it. When I go uphill or the battery is low, I can hear a distinct engine pinging or noise. It’s different from the usual sound, and it mostly occurs when the hybrid battery is running low. I’ve looked through some of the old posts, and most of them are either for the Gen 3 model or engine noise while idling. However, for me, it’s been when I’m going uphill and the battery can’t assist much that the noise starts. Usually, I ease off the throttle, but even then, it persists.

I’ve tried using 91 octane fuel, but that didn’t make any improvement. There’s no check engine light. The spark plugs were replaced 30,000 to 40,000 miles ago, and they’re iridium spark plugs. Both the hybrid and engine coolant have been changed along with the transmission. So, I’m wondering what else could be causing this issue.