engine slight knocking

I just bought a 2017 GMC Sierra CK2500 few months ago. i Had RAM and Expedition before.  They all slightly knock specially when driving up a hill. My question is why they all does that?
Ann KAsked:
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dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.Commented:
Probably under load.  If you are lugging the motor up the hill (making it work in top gear at low revs) you'll probably be getting slight knocking at times.  Low grade or low octane petrol can also cause this under load.
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RaminTechnical AdvisorCommented:
If your car has engine knock sensor it is very likely the knock sensor is defective.
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Jackie ManIT ManagerCommented:
At most retail gasoline stations, three octane grades are offered, 87 (regular), 89 (mid-grade), and 91-93 (premium). The higher the octane number, the more resistant the gasoline mixture is to knock. The use of higher octane fuels also enables higher compression ratios, turbocharging, and downsizing/downspeeding—all of which enable greater engine efficiencies and higher performance. Currently, high-octane fuel is marketed as ‘premium,’ but automotive manufacturers have expressed interest in raising the minimum octane pool in the United States to enable smaller, more efficient engines. Doing so would increase vehicle efficiency and lower greenhouse gases through decreased petroleum consumption.

Source: http://www.eesi.org/papers/view/fact-sheet-a-brief-history-of-octane

Are you using fuel of octane value of 87 or lower?
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JohnBusiness Consultant (Owner)Commented:
As others have noted, regular gasoline is a common culprit. I use 91 octane in my non-Turbo Volvo and 94 octane in my Turbo Volvo wagon. No knocking.
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