It could be spark knock.
It could be a something vibrating.
It could be a lot of things.
I filled my "new to me" GT tank with 87 octane when I picked it up. Unknown to me the car had been apparently retuned. The car lost power, and would rattle when accellerating under a load.
I ran the car down to a half tank of gas and then refilled it with some VP 103 octane racing fuel that I had, and I poured an octane additive. Once the car was empty I filled up with regular 93 octane. My spark knock went away.
If you have a manual you have to remember to downshift for big hills. The extra load is just dumping fuel into the cylinders. You will end up clogging catalytic converters, and possibly destroying pistons and cylinder walls due to detonation. If spark knock is bad enough you can blow out plugs, head gaskets, or even destroy a head.
Run a higher grade of gas and see what the results are.
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