If it says recommended 91 then it is only recommended not required so you can put 87 in it just fine with no long-term effects ChrisFix has a video on this Heres is the video. The ping would most likely be detonation but if it says recommended 91 but says you can put 87 in there I don't see why it would ping. could you add a video of the pinging noise if you can catch it? But if it is detonation then that can cause serious internal damage to the engine

They usually have a knock sensor, which will do the adjustments for 91 or 87 octane. If you want to save money, use 87. If you want performance, use 91. That’s pretty cool it gives you the option of either. Some high performance vehicles only use 91.